Get GTR2 and/or Race 07 (plus various expansion pack(s)) - you will want some sort of steering wheel for PC sims though.
GTR2 has events to unlock tracks. There's not much in the PC-specific space for buying/unlocking/collecting cars. NFS Shift games probably come close. As do the DiRT and GRID series, although you may already have played them on console.
If you really want to grind for cars, try out SimRaceWay and/or Auto Club Revolution which are free. They encourage you to spend real money on cars but they can be unlocked for free by grinding. Personally I think SRW has the best model. RaceRoom on the other hand doesn't let you do much more for free than a demo for a full game. GRID Autosport is 75% off on Steam today, but despite purchasing it months ago I've never played it so can't offer a review.
So my two picks to start off with are SimRaceWay (free) and GTR2 (very cheap on Steam). Put the money towards a wheel such as a Logitech DFGT or G25/27 on eBay if you have limited funds.