Fair enough, although a livery ( or collection of ads ) would mainly serve as "wallpaper" without a meaning attached to it in my opinion or maybe people can imagine being sponsored for all I care, my main point was it would add a sense of realism to some ( me included ) or just being fun creating a livery ( I wouldn't use a livery editor to replicate existing ones myself, I'll happily leave that to PD ).
In general ( meaning road cars ) I share your preference for a clean simple look, but when I fully tune a car into a race or rally car I would like to "put some stickers" onto them to make clear they are in fact race or rally cars and not a tuned version with a hideous rear spoiler, etc.
And besides a livery can be subtle and although there are quite a few exceptions I do like, in general who does like ads?
Getting back on-topic, I think the Clio V6 may actually be a good starting point for a rally car or how about the VW Sambabus?
Actually, part of having a successful racing career implies being backed-up by wealthy sponsors (just check out the recent news, with a young mexican driver hired by Sauber, helped by his support from Telmex). While I think it could be fun to have a "sponsor hunting" part in GT5 in order to enter some competitions, I understand that it would be bothering to most.
On topic though: Clio V6 is a good choice. Personally I've been thinking of basing a rally car on a BMW M3 coupe, though it would be preferable to swap for a 4WD drivetrain.
...I remember when I had my old 240; 
several people have had fun with V8 conversions. If only in GT.
), Time Attack 4WD + 2WD and Vintage and possibly Pikes Peak Open depending on how much we can modify cars in GT5.
