rcas are called PRE OUTS... since you are gonna be running a sub and door speakers you need to decided how many door speakers.... i'll give you a few scenearios and some suggestions... i'm doing this right now also (picking a new head unit) but i'm going a little higher end however i hope this helps...
If you want to run 2 door speakers, 2 rear speakers and the subs you'll need: 3 sets of pre-outs with one set being a non-fading pre-out or sub specific pre-out. You'll also want built in crossovers, look for HPF and LPF in the specs of the deck
If you want to run 2 door speakers and the subs you'll need just 2 sets of pre-outs but you still need the non-fading set and the crossovers
To really benefit from something sounding good make sure you have the crossovers thoses there will make or break the sound quality. Also Pre-out voltage is important, unfortunately Pioneer decks are noctorious for claiming one thing and being way below their claims, so if it says 4V pre-outs, you should expect 1.5-2v from them.
Other features that are nice but not necessary are built in EQs and EQ presets, CD Text abilities (this is nice if you burn your own cds and write Text to CD), now don't get CD Text mixed up with the ability to specify a name. CD Text atually reads the text on the cd and displays it, the other way you need to punch it in (kinda dumb). Other nice features are the ability to specific the db level for the sub or non-fading pre-out, and to change the actual crossover points. If you are into in screen savers are cool but don't be fooled by them, they don't make a deck sound better.
What do i suggest, i would say to stray from pioneer, the decks are jammed pack with features but compared to other decks they sound like ****.... the high end sony decks (mobile ES line) are very nice and most are under 300$ and they come with all the features including dancing graphis on some models.... the L3 line has a very unique way to control the deck also, no buttons on the deck, instead you get a wireless joystick (and remote), you mount the joystick anywhere you want and control your deck with it, pretty neat idea. Other companies to look at are alpine and eclipse, although expect to pay for an eclipse deck.
If you need more info just let us know, but i hope this helps you a lot.