The Beseler is a good choice. I have one and they're a work horse. I also use
it for Bronica 2 1/4 which it has a negative holder for. I don't know about
the Pentax but the Canon AE1 is also a good work horse camera offering
interchangeable lenses, and manual override on exposures.
I recently got a catalog from B & H Photo in New York City which when I
received it looked bigger than the Encyclopaedia Brittanica. Their Phone
number is 212 444-6630. Believe me, they'd be happy to send you a catalog.