Without even a ballpark guess about your budget, i can't say much; I would, if I were you and fixing up the beater, get a rigid fork (Surly Instigator for me, a Clyde; it runs about $80+shp). $65 for a good tune-up would be money well spent. Might save you $$ on parts you think you need.
I have a similar project on the back burner -- an old aluminum hardtail I'm converting to a rigid singlespeed (hence, the Instigator fork). Gonna run me about $400, since I have only a bare frame right now to work with...that's why it's on the back burner! Kids kinda come first....