I've been building my C330-based system for a while and I must say that your $300 budget will be a challenge. Mamiya TLR lenses tend to develop "lazy" shutters due to old lubricants and most of them can benefit from a CLA. So you either need to find a camera that has been used recently (most likely had been serviced) and those tend to be more than $300, or you'll find an old "shelf queen" and then you won't have budget left for CLA. Clean and serviced C330 systems with lens hover around $500-750.

With your budget I would be looking at Minolta Autocord instead. In some ways it's a nicer camera because it's more lightweight. A Rolleicord would also be a nice choice, and probably a few others suggested by the fellow redditors here.