If you want to get a stock for building internal structure and the color isn't important, pick one which is cheaper. Blue is a good candidate, red isn't. Black should be OK too.
As for the sizes, there's no real way to answer that, but if building 2 studs wide, 2x4 is the one you'll need most, with a dash of 2x2 for ends. They are usually easiest to find and cheaper than longer ones.
For 1 stud wide building, you'll of course need some 1x1 for ends, but using only 2x2 can be cumbersome unless you want the effect (try a well in 1x2 dark red plates, it's superb), so then you'll want to stock on 1x4, 1x6 and 1x8. Again, check the prices per stud.
But if you're willing to buy big quantities and are after good prices, the simplest is to by full tubs - and then you don't need to worry about which ones you'll get as the assortment is fixed. You may also want to check specific creator sets for clearance (houses are a good source of 1x bricks).