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Stars on a driver

Hello,

Yes. 
You can only purchase drivers (or staff) that have a rating lower than or equal to your current manager level. 

It is possible for your driver to exceed your manager level if you train them - however training progress beyond this point is much slower than normal training.

Hello,

For drivers, it is the stats that are important. 
Unlike staff, the number of stars a driver has doesn't impact their performance.

For drivers, all except two of the stats are trainable. The two stats that you can't train are Talent and Experience.

Experience is gained as the driver completes races, so although it's not trainable, it will improve the more races a driver competes in.

Which just leaves us with talent. This particular stat is probably the most important of all, it can't be trained and will never change. So, you can pretty much disregard level and star ratings (which, at the end of the day are just a measure of the difference between team level and driver level) and focus entirely on Talent. As a long term investment, in performance terms, a high talent driver will repay any investment you make.

High talent, low level drivers are very rare on the market, so keep looking at what drivers are available in your Youth Academy which refreshes every couple of days.

As you develop your academy there is a higher probability it will spawn a high talent driver.

Isn't that what I said? Experience is gained as the driver completes races

People considering the use of Sprint Races to increase driver experience should be aware that SRs will wear their car. No parts or engines will be earned through SRs if the team is above Level 5. Now that Ads are disabled, any parts and engines that are required over and above those that are gained from daily rewards will need to purchased with tokens.

Additionally, SRs will make drivers lose health which will reduce the speed at which the other attributes can be trained. SRs will also result in drivers ageing which reduces the length of their League Race career.

Transferring drivers to secondary "training accounts" then later moving them back to the main account could result in these drivers being removed from the game with no compensation as it is considered an exploit.