Tamiya acrylics are excellent for airbrushing, but a full set seems ambitious. There's alot of colors I think I'd never get around to using.
The gloss acrylics can definately be used for painting bodies. They're not very popular around here as they are softer than laquers- but with some patience and good technique, you can get a terrific shine from them. Theyre also great for custom color mixing. My 996 was done with Tamiya acrylics, as well as my Z3.
Also check out
Gridgirl's New Beetle in progress. (I expect they can also be used on models of non-German cars, too.)
X20A is a great option to thin it with. Get one of the large sizes.
I never thin to a ratio- the amount of thinning required will vary with nearly every application. I just add a bit of paint to the color cup, and add a bit of thinner with an eye dropper, test spray and adjust as necessary.