Figuring out if you should buy or make a costumes depends on how much experience you have with making costumes/ sewing. It also can depend on if you have someone who can help you or not. It sucks when you start a costume and get frustrated because it turns out to be too complicated. If I were you, I'd think about my skills and how complicated a costume I can do, and then decide if it's possible : )
The point you bring up about it fitting is also very important, but usually if you buy from a good website with good reviews, and you pick the right size then your costume will fit.
I'm not sure on the cost, but if you can't make it, buying it would be better : )
and for the glasses, you can pick up reading glasses at the dollar store and pop the lenses out.
Hope this helps : )
