I wouldn't really see your suggestions as fixes. I don't know, I didn't design the credit system or the balance of the single player, but it makes sense that DLC follows the same format as the rest of the game, i.e., you spend your cash wisely based on your taste and preference in how you want to build up your garage. With our next title update, you'll receive 150k credits per DLC pack to put towards a car if you wish (I need to check if this is retroactive or not).
Those of you who want to simply collect every single car ever, yeah, it's going to get prohibitively expensive. The system is there so that the cars are trickled out based on your reward earnings, and hence, the "progression" inherent in a racing game like Forza. Like Psychotext said, you can test drive, play online, and take photos of every car in the game. If you want to unlock more access to these cars (i.e., "own" them) so you can then paint and tune them, yeah, you'll need to play more of the game. Why should this be seen as grinding? You earn credits and XP for any racing you do anywhere in the game (outside of hotlapping).
Taken to extremes, you could say that folks pay $65 bucks for the game so why should they have to pay in-game credits again to own each of the 400 cars?
There are more ways than racing to earn credits in Forza. Playing multiplayer, selling stuff on your storefront, selling cars in the auction house... hell, you might even be able to find some of these DLC cars eventually on the auction house for cheap.
Glad folks are enjoying the 458 Italia. Absolute joy to drive and behold.
BTW, we should get some nightly (or afternoon, whatever) GAF games going.
Edit: Holy shit Jube!