definitely get a 2 button, its classy, traditional (but not dated) and will still be in fashion in 5, 10, 20 years time. Depends what it is for, as to the colour. Dark for formal wear, navy or grey or even black. Sleeves should show half inch of shirt sticking out. Good fit is essential, many off the rack suits can hang wrongly unless you are very very lucky. Consider a tailor if you are willing to pay more.
Trousers should rest nicely on shoes, with just a single ruffle in the crease. No turn ups for the trousers. No pleats for the trousers either.
If tailored your probably wont need a belt as you can choose not to have belt loops, but off the rack suits will have loops, its no drama. Never wear braces.
material depends on the climate where you live, wool is normally good for most climates, the quality of the material varies widely. Not sure if i can remember the numbers but i think decent material starts at about 100 (its some kind of measure of quality of weaving) and 140's are great, about 200 or so you are talking very expensive stuff.
On the cuff, 3 or 4 buttons are fine, they will be sewn on, but tailored suits will have real button holes, off the racks wont.
as for vents, i prefer single or double vent, either is classy, Armani do no vents, and its quite the italian thing, but not my taste. Try to get a look at how the suit hangs on your back, that will tell you if it fits well, it should hang nicely, with no folds or gathering of the material.
for 2 buttons only do the top button if you need to do a button, never both, and when done, there should be no creases or gathering of the suit at the front.
