Crank recommendations?

Hi, in the process of converting my specialized 90s hardrock to a commuter for campus. Plan on ditching the current triple crank\deraileur\shifter for a single speed on the front (for now). I don't leave my second ring as it is and wouldn't mind dropping the weight (although very little). Basically just tidying up and tinkering. 7cogs on the back but will likely stick to 2-6 as to avoid dropped chains.

Trying to keep it cheap and so far have only found one good candidate being the truvativ hussefelt 1.1 (38 tooth). Will go with a 170mm length if I can find it just for kicks (never had less than 175).

A few questions;
1) Should I go with a smaller crank if I eventually plan on going full single speed? I am mostly clueless as gearing combos. Especially a mutt like a single speed mtb commuter.
2) Other crank suggestions?
3) Would a bmx crank be a wise choice and where should I look for one?
4) Bottom bracket seems ok, any trouble installing a new crank?
5) Terrible idea?

thanks for any help!