I've got two Trek 700s (had a third but sold it recently) of 1990s vintages and I've put Shimano UN54 BBs in all three. The stock configuration left plenty of room that a 1-2mm change in either direction wouldn't cause any trouble. Yours may be an older model being it had a cup and ball set up but I am confident that either the 115 or 118 mm spindle will work just fine. Your decision will affect the chainline by about the width of one rear cog, so if you like using small cogs with the middle chainring, look at the 118. If you ride mainly in your big chainring, I'd pick the 115.
Though not the lightest framesets around, I'm a fan of the 700 series for quality of ride and bulletproof durability. One nice upgrade to a basic 700 is to find the fork off a 730 which is chrome moly. It's slightly lighter and has a bit more spring to it. The stock fork is fine but you will notice a reduction in fine road vibration with the 730 fork. You can get aftermarket chrome moly forks but they don't quite match the original and always look like a replacement, a fork from a 730 looks stock.