Seems like people are really struggling to enjoy this beautiful game, so here's small guide to help you guys understand the game progression. Before I go through the progression path, I'll make a few key points about the game's mechanic.
1. THE REP LEVEL MATTERS A LOT. The level cap is 50, but this by no means that level 30 is a significant level. The Rep Level allows you to unlock the progression for engine components, engine swaps, and higher level cars. This should be your primary concern.
2. You can make money racing any time, but if you're trying to bank a huge stack before you finish leveling, you are putting all of your money at risk every time you go into the night mode (where you level). You should keep your cash balance minimal to avoid the butthurt if you get busted.
3. STOP GETTING HIGHER HEAT LEVELS THAN YOUR CAR CAN HANDLE. You are easily able to outrun the cops if your car can pull a stretch of road above 170-180 m/h.
4. Your Rep level determines your reward for doing races. The difference between coming 1st and 3rd is very small, but the difference between coming 1st at Level 20 vs coming 1st at Level 25 is very big. The higher the Rep level, the more money you get.
My personal progression, in terms of cars, looked something like this:
Starter: Nissan 180SX (doesn't really matter)
Infinity Q60 (until Mercedes C63 unlock)
Mercedes C63 (until Corvette ZR1 unlock/money to buy)
Corvette ZR1 (until endgame, or any other supercar)
Ford GT (max level, all cars owned)
The idea for the car progression is that you want to focus on speed, as you can easily surpass AI players (even on Hard) if your car dominates in speed, even if you lack in turning capability. You make up the time loss on the corners by going faster on the straight roads.
I would recommend this sort of focus, even if it is against your desired playing style, as you can endure a bit of pain and suffering for the longer term benefit. Once you finish leveling up, you can get whatever beater car you want and customize it to your own desires.
I, personally, avoided any of the story or side missions until I got to Level 50. Wasn't interested in the story, but also the missions did not appear rewarding in terms of money and xp gain. However, this is purely a choice of personal preference.
Guide to Leveling and Making Money:
1) Leveling (to Rep Level 50):
Spoiler
The main area of focus here is the Palm City Speedway area, where the default garage is. At night, you are able to go on the Speedway and drive around the track. This here is the fastest way to level.
You can drive clockwise around the loop, and before coming into the 2nd stretch of road that goes opposite of the starting direction, you must turn off into the stretch of road (within the loop) where you see the mobile speed camera event. It is a 3 star event, just as all others, where you earn all 3 stars if you pass it going over 170m/h. Every time you pass through it at night, you will also receive points towards your Heat level.
For reference, a loop of this track should take you anywhere between 15-20 seconds at max level and up to 30-40 seconds at lower levels.
PLEASE USE A DAMAGE REDUCTION AUXILIARY PERK. THIS WILL SIGNIFICANTLY IMPROVE HOW MUCH DAMAGE YOUR CAR CAN HANDLE.
At Heat Level 0, you will earn 1.7k xp every time you pass this checkpoint. It will take you 4 passes at 3 stars to get from Heat 0 to Heat 1, where your xp multiplier becomes 2x. At this point, you will earn a little over 3k xp per pass on 3 stars. After 4-5 passes, you will get from Heat 1 to Heat 2, where the xp multiplier becomes 3x. This will earn you about 5k xp per run (1.7k * 3). So on and so on, until Heat Level 5, where you earn 9k xp per pass with the 5x multiplier (1.7k * 5). This amounts to about 200,000-250,000 xp every 15-30 minutes, depending how fast your car is.
You should keep running this circuit in a loop until you rack up enough desired xp. Once you have gained enough levels and want to cash them in (to prevent losing to the cops, who will chase you around the track), you can turn around and go counter-clockwise on the track. Here, you will want to cut halfway through the track and take the jump ramp that takes you out of the circuit, in the direction of the Parts Shop (on the map). You will land in the proximity of the alley way, where you can simply drive forward (since you lose the cops during the jump), and hide on the field until you escape. You will cash the points, level up, and repeat the activity.
I would recommend cashing in every 4-5 levels (or 200,000-400,000xp), don't get greedy. If you get busted, you lose 40% of ALL your money and nearly 100% of the xp you gained that night.
In terms of cars, I would highly recommend just using whatever your fastest car is. If you're fancy and have the Deluxe Edition, you can use the free Corvette Grand Sport, which is surprisingly fast and effective/efficient for this method.
Keep doing this until you get to Level 50.
2) Earning Money:
Spoiler
This is a relatively simple task. Once you get to higher levels, you will make money just playing and having fun, so it becomes less of a concern. A key thing to note is that when doing racing events, you will make incrementally less money doing the race over and over again if you do not reset the day (only applies if you come 1st). If you come 2nd in your races, the prize bonus for coming 1st does not change, thereby not reducing your income of coming 2nd. You could, theoretically, keep coming in 2nd and make more money relative to coming 1st, as you do not have a decline in winnings.
However, during the early and late progression, the best place to race is again the Palm City Speedway, right outside of the default garage. This is a 4 lap race around the circuit, where the average time to completion can be anywhere just over 2 minutes (Mercedes C63 at level 25, with ~600bhp) down to 1:35 per race (Ford GT maxed, ~1200bhp).
Essentially, you will want to run this race twice, if you win both times. This will give you the reward for doing the race, as well as the smaller reward for doing the race a 2nd time (about -30% less in winnings). Once you have the 2 races, you will teleport back to garage, enter and exit garage, return to the track, and rinse repeat.
3) Unlocking Ultra+ Parts:
Spoiler
Keep doing the same tactic as the Leveling in the Speedway, where you run the laps until you reach Heat level 3 or 5; this is the fastest way to gain Heat level. At Heat level 3, you can go and do the High Heat Level 3 race. As far as I've seen, there isn't much difference between Heat 3 and Heat 5 races; they use the same AI cars with the same difficulty, and have similar rewards, although at Heat level 5, your RNG chance of Ultra+ is higher (but not that much, really).
As you progress and become faster, you will want to do both events every time you enter night, as it will save you having to do twice as many progressions to the Heat levels.
Note: if you get busted while you having a volatile part upon completion of a race, you keep the part.
I hope that this is a helpful guide to some of you. It has certainly served me well and I have been very much enjoying the game, even beyond my completion of it.
I'm sure I've also missed a few things, but if there are any questions, feel free to ask.
