* Go with a solid staple color, my order of preference: navy, navy, navy and medium grey.
* 2-button or 3 roll 2 are fine.
* Notch lapel
* Functional button cuffs are a very nice detail - only if the sleeve lengths are spot on. Otherwise it becomes a hassle to tailor and you should buy one without functional cuffs.
* Both center and side vents in the rear are fine, although center vents are less common these days.
* Most important: Shoulders should not extend past your natural shoulders. This is the most important element in terms of fit.
* Jacket length should land about at the knuckle of your thumb
* With top button only buttoned, should be snug but not tight. Keep sizing down until you get the "X", and then go back up a size.
* Armholes: Lift up your arms - you shouldn't see giant wing-like things underneath them.
All of the other details are nit-pickier stylish things that most people won't care about. The other thing you'll have to consider is the quality of the suit. You basically have a few ranges:
$200-$300 range: These will be fused (glued) suits. Basically a starter/throwaway suit. If you have the money, consider going up to the next level to try to buy a "suit for life". The other downside is fabrics here tend to not be as nice. From what I've read some decent slim options are the Bar III suit at Macy's and the JCrew Factory Thompson suit.
$300-$400 range: Some JCrew options here that are decent and half-canvassed instead of fused
$500-$600 range: Good options here are Suit Supply, Brooks Brothers 1818 line (fitzgerald, milano) , half canvassed, or something like the Benjamin suit which is fully canvassed. Fabric will be better quality like super 120's, 130's, etc. At this price range you can also get some real solid quality during sales, such as Polo, Ralph Lauren black label.
$600+ Probably not what you're looking for, but you'll get better little details.