The one really terrific invention on the paint front over the past decade or so is the arrival of the electronic scanning machine at the paint store that can identify your original color from a sample.  In the old days, you might spend an hour looking at chips and trying a few formulas, and never quite nail it.

Paint is more-or-less broken down into two types, latex and enamel.  In the old days, latex was usually flat and absorbed every smudge and fingerprint like a sponge, so enamel was used on interior doors, jambs and windowsills.  As latex has gotten better over the years, with glossy, hard-shell coats, the need for enamel has shrunk.  It's really only used for 'projects' these days.

Paint is a classic case of 'you get what you pay for'.  The cheap-cheap stuff will be so thin it might actually take an extra coat to cover the old paint, so you end up saving twice as much money and doing twice as much work.

On the other hand, I've bought real expensive, thick "1-coat" paint and a few places still looked they needed touching up later on, so I think the best advice is to buy medium-grade paint and expect to roll it twice.
 

Stain is fairly self-explanatory.  Just be prepared for some wild variation in shade (not color) from the example chip at the store and what it looks like in real life.  The type of wood it's being used on makes a huge difference.  In general, the wood they use for store displays is fairly dense, whereas chances are you might be working with some pine or redwood, and it'll suck up the stain like a sponge.  If you're worried it's going to turn out too dark, thin it with the proper thinner.