You should be able to get into a late C4 with reasonable miles for well under that 20k
The market has tanked and they're just not selling like they used to
The C5 is a great car as well. The ride is going to be head and shoulders above the C4 and the performance will amaze you
As for driving it home, thats not a problem. If you get it from a dealer, you;ll get a dealer tag. If you have your bill of sale, you're good. You can get a temp tag from your local tag office until your new one comes in unless you're using the one off the other car?