Okay, let's start from progression-related tips.
1) Trade unneeded speed cards in instead of selling them. The payout for selling speed cards is not too high, and once you reach high rating on your car (especially the 399 rating), tokens will become way more important than cash, as rolling for cards with tokens is the fastest and most reliable way to get speed cards with right perks/brand.
2) Don't focus on too many builds at once if you're short on cash - Generally, cash-wise the smartest idea is to do the following: Get your starter car as close to its max level (299) as possible, and once you progress far enough to unlock cars with 399 max level, buy one for each class and stick with it all the way up to 399.
3) Take the suggested car level in race description with a grain of salt - In most of cases, you can win just fine with a car that has rating 30-50 points lower than the recommended one. I'd say that you need to be the closest to recommended rating in late Race car races. Drag can be a bit tricky too, but it's doable. When it comes to drift, you can manage just fine with whatever starter car you have regardless of level - the only thing you really should care about with singleplayer drift build is nitrous. Sure, once you complete the game you should upgrade the cars to fight for better times/results, but in terms of SP races alone you don't need to trust recommended level too much.
4) If you want to upgrade your car fast, I recommend doing the following (it requires a bit of cash and tokens though):
- Buy out speed cards from the shop that would increase your rating without caring about perks or brands - the only point of this step is to get your rating higher. Higher rating = higher race rewards and better cards in the shop once it restocks
- Once you do that, go and do Midnight Run a few times - it's the shortest race, so it's perfect if you purely want to obtain cash. If speed cards from victories improve your rating, equip them. If not, trade them in for tokens.
- Once shop restocks, buy cards up again (money from winning races from point #2 should mostly cover that), trade old ones in for the tokens, and then keep on repeating 1-2 until you get to 18-rated cards
- Once you get to 399 rating, use the tokens to get the perks and/or brands you want. Brand-wise, I usually go for Chidori and/or Americana on Race builds.
Now, for other things:
1) Regardless of the car you own (although this tip is especially important for the Regera), make sure that your final build has nitrous maxed out. Nitrous is critical for your end result - it will let you get out of chaos at the start faster, lets you get your speed back faster if you crash and is pivotal for accelerating out of turns.
2) In terms of live tuning, always prioritize settings that will improve your speed, acceleration and car's reaction to turns. It doesn't make any car significantly more difficult to handle, and at the same time it helps you gain at least a couple of seconds during the race.
3) On Regeras, if you really want to beat one online and you don't have one yourself, vote for tracks with a lot of turns (if there is one thing Regera struggles with, it's acceleration) and try to get physical at the early stages of the race - if you have a 399 build, you're practically guaranteed to overtake the Regera at the start. When it tries to work its way back to the front, bodyslam it - that's the best chance you will get, and Regera is one of easier cars to knock out.