Elfenstar, if the weather's cold (at least fridge temp) and you leave a carved pumpkin outside, it'll last for up to a week before it collapses in on itself due to rot. If the weather's warm, that could happen within a couple days. The lit candle inside the carved pumpkin also contributes to its downfall after a couple nights, but makes for a glorious scent of sweet cooked pumpkin. An uncarved pumpkin will last a pretty long time indoors (not v. warm) or outdoors - months, for the latter, unless it gets water/condensation trapped underneath, in which case rot ensues pretty quickly.
The best carving pumpkins are generally the tall ones rather than the squat ones. You only need one good (flat, unscarred) surface for the face/design, though, so feel free to pick one that is a little slanted or dented as long as it has that one good surface and it has a flat base so it won't roll or lean. (Rolling or leaning pumpkins can be a devil to carve and can be fire hazards if you put candles inside.) Squat pumpkins are harder to carve in that once you remove the lid there's not much space left on the sides to work with. But if you're keen you'll manage that, too. Just don't pick a pumpkin that is mushy or that has injured flesh anywhere, as it won't last long. Scars are fine, as long as they've healed over. Ideally your pumpkin will have a stem still attached to the top, with which you can lift up the lid once you've cut it off.
Those pumpkin carving tool kits that are sold at some supermarkets here this time of year really are handy - much much safer than using a sharp knife and the save-like (but non-sharp) serrated tools especially allow IMHO for much more accurate cutting/design reproduction in hard pumpkin flesh. Good tips and pictures of carving tools are here:
pumpkin advice