If you're planning to buy off the peg and alter (definitely the way to go unless you're very oddly shaped or want the full bespoke experience) then concentrate on getting the shoulder fit right - that's the hardest to correct later. If your shoulders are right, but it won't button up, go looking for another cut of jacket. If you go up a size on that one, you'll end up with dodgy shoulders and you really don't want that. If the shoulder is right and it buttons up, but there's an 'X' coming from the button (Three buttons? Middle button and maybe the top, never the bottom. Two buttons? Top button only. The bottom button should never be fastened) then see if you can get a feel for how generous the seams are - a good alteration tailor might be able to sort you out.
Even if everything else fits, you'll probably want the sleeves shortened - you want to be showing about half an inch of shirt cuff. I can't remember the last time I saw someone in an unaltered OTP suit that had sleeves that weren't too long.
A good check for the shoulder is to lean against a wall - if the shoulder pad hits substantially before your actual shoulder and especially if it rides up, then that's not the fit for you.