Okay. I'd definitely suggest going to an actual store to try on suits, but if you find one you like and you can get the same suit online for cheaper, there's no harm in that. That said:
1) Your suit does not need to be expensive to look good or to make you look professional. I've seen cheap suits that looked stunning, and pricey suits that looked like utter crap. Pick one that makes YOU look good.
2) It doesn't need to be perfectly tailored and fitted (unless of course you need the tailoring for it to look good: see #1). Does it fit you pretty well? Do you stand in front of the mirror and say, "I look good!" Then don't bother with tailoring. You'll gain weight in med school, and that lovely interview suit won't do you a lick of good.
3) Regarding weight gain: think I'm being pessimistic? Get to third year. Try waking at 3 AM, eating breakfast 'cause you're hungry, then being hungry again at 9 AM (and when the team goes for breakfast, you just can't resist), then being hungry at 2 for lunch, and then being hungry at 8 for dinner, and then you're on call, so by 2 AM, you're famished again. It's the 4-5 meal a day plan. And unless you're dashing up the stairs to the 10th floor, you're probably not going to have a lot of time for exercise. So don't justify a really expensive purchase thinking you'll be able to use it again. You may not be able to.
Bottom line: buy a suit that you look good in, that you're comfortable wearing for extended periods, and that you can afford. I went to Macy's, and that worked well. A couple of my friends went to Ann Taylor. Those are really good places to start.