There's nothing wrong with a non-matrix meter, you just have to be aware of what it's doing. If you're shooting a backlit subject (say, against a sky), adjust upwards. About the only situation where it might make a difference is shooting transparencies in tricky conditions, otherwise the negative exposure latitude will easily cover you.

Pentax 6x7s aren't that bad, a non-MLU body, a TTL prism, and a 105/2.4 or 75/4.5 should run you around $120 each. But a 645 and a 6x7 are very different cameras. The 645 will shoot like a 35mm SLR, and the 6x7 will probably be a bit slower but with a much larger negative.