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Right off the bat, I would say, "Forget the C3". Lousy ride and handling. I'd say C4, but I'm biased, as I own one. You can get a really nice C4 for $20k. C5's are technologically better, I suppose, but they're ugly. They do have a big butt--for luggage, I guess.
If you abide by the "Buy the newest car you can afford" theorem, then, it will probably be a medium-to-high mileage C5. But, I would look for a $15,000 C4 and save the rest for repairs, modifications, or goodies. Whatever your decision, enjoy it and have fun.
For $20k, you are talking 2000+ C5's. Maybe even a couple Z06's. The only C4's in that price range are the "collectible" cars like GS's, some 40th's, and ZR-1's. If you are talking regular C4 that you just drive and have fun with, you could get into something nice for half that amount. So for a fun driver, a C5 at that price point sounds more sensible.
Spend money on a C5 and the day after you make the purchase it will be worth less than the day before. If I had that kind of cash to spend on a toy, it would be a chrome bumber C3, GS C4, or 40th C4.