Without fail, the first thing to look for is the SFI rating. Just about every catalog, website or whatever will list the SFI rating for every suit they sell. An SFI 3.2/5, sometimes listed as just SFI 5, is generally a two layer suit, and down here that will make most organizers happy with or without nomex skivvies. Some will let you use a 3.2/1 single layer, but only if you wear the fireproof undies. Like Woob said check with whichever club you plan to run with to see what they require. Regardless of club requirements nomex (or carbon) long johns are cheap insurance should things go terribly wrong and they do help keep your suit from smelling like a locker room.
Once you've found a suit that meets your club and personal safety requirements, then add the extras to your heart's (and wallet's) content. I agree with everything Woob said about comfort. Little annoyances become huge when you can't adjust. That little crease under your shoulder belt will start to hurt after a few minutes and you won't be able to do anything about it until your session is over. The extra cost of some comfort items is money well spent, in my opinion, and while you don't need to spend $1500 on a suit, the ultimate budget job will probably fit like... well... a cheap suit.
Having said all that, we all know the primary purpose of the driving suit is to look incredibly cool at the Denny's for breakfast on the way to the track. Chicks dig race drivers...