What you are likely to end up needing is TWO air compressors, so start small. Here's why:
A portable compressor is great for nailguns. And it's, uh, portable, so you can take it to the jobsite. Helpful tip: if you buy one that doesn't have wheels, buy an inexpensive dolly and bolt it on. Once you lug a compressor a couple times you'll understand. HD has a great plastic-framed dolly for about $30; I bolted my Porter Cable compressor to it and now it's EASY to move around. Portables start around $200, but get the biggest one you can afford; you'll be glad you did.
BUT: portable compressors don't generate anywhere CLOSE to enough CFM to run a paint sprayer, die grinder, or pretty much any 'continuous flow' air tool. So you'll eventually need a bigger compressor. A 7 Hp 80 gal model is a huge beast; I'd LOVE to have one but most folks don't have the room or the $$$ for such a monster. The most common 'larger' size is a 3 or 5 Hp, 60 gal tank. They start around $500, but price varies a lot due to different configurations (single-stage, dual stage, one/two/four cylinders) and quality levels. Retail stores like HD and Lowes carry 'residential' quality units; if you want a bad-axe-mother-fletcher compressor you'll have to find a shop that specializes in industrial-level gear. And check the repair places too. There's a place here that repairs compressors, and they sometimes have deals on used equipment. You can get a lot higher quality/capacity unit if you don't mind a few nicks and scrapes.
Happy Hunting!
c.