I'm in a similar boat!
I just got a Mamiya 645, 4 lenses different size film inserts and a couple of other things for it... It's really great so far, though the mirror is getting caught on a seam of foam that has risen, probably an easy fix.
Anyway, I wanted another incident meter, as I had a great experience with my Weston Master II and invercone, though the selenium crystal seems to only work in bright outdoor conditions in its very old age.
So, I got a Gossen Luna Pro w/ a 7.5/15 degree spot meter attachment and a Gossen Luna Pro-F (for flash readings) for around a hundred bucks altogether and both of which seem to work really great (and both accept the spot meter attachment).
I'd love to have a pentax or minolta style spot meter, but this seems like it will work just great for metering a subject or area... Both Gossens are plastic, but a very heavy duty and sturdy housing, still. And the Luna Pro -F takes a normal 9v battery instead of weird discontinued mercury batteries like the luna Pro (I bought expensive adaptors to use normal silver oxide batteries with it). Plus, I snapped up the Luna pro + spot attachment for like $80 and then the Luna Pro F afterwards for $20... Pro-F also has a cool design- instead of telling you where to set the dial like my Weston and Luna Pro, the Luna Pro-F moves the needle and tells you which way to turn the dial until the needle is at the "0" position. Pretty cool, especially for $20 and flash metering...