Used system for about $250?

[srs5694]

The Pentax should do nicely, from what I know of it, as should something from the Mamiya 645 series. Your need for a light meter rules out some otherwise good options, like the Mamiya C-series TLRs, and the need for interchangeable lenses rules out other TLRs, folders, and (AFAIK) all MF rangefinders. You could of course always add a separate light meter -- basic ones are very inexpensive used, although fancy ones can cost upwards of $100, which would eat seriously into your budget.