The only thing on the 318 is the small case diff, which should be good to 250-300hp, lots of people run the small case diff with no problems though
If its a small case LSD as its harder to find you can sell for the same or more then a medium, so it's a wash on price.
Like said before, go for a solid shell, anything beyond structural parts are easily changed and parts are not overly expensive.
I'm running the m20 this season, and maybe next, after I get the suspension, and cage in, I am planning on a 302 v8 swap. All parts $3000 and should have 350hp at wheels natural aspirated.