Portraits using adapted lenses might be tricky (but certainly not impossible) given that you'd be limited to using the electronic shutter, which takes about 1/3 second to sweep through an exposure (plus the exposure time itself). With this limitation aside, for longer focal lengths you might look into the older Hasselblad Zeiss V system lenses. These were designed to cover the 6x6 format so will do well with the smaller 33x44mm sensor. In my experience, the 100/3.5, 120/4 Makro, 150/4 and 180/4 are all excellent - good copies in the "CF" mount can be found for under $1000. The 250/5.6 Superachromat is exceptional; good copies are a bit harder to find and are pricier ($2500-3000).
The native XCD lenses in the comparable focal lengths are almost certainly sharper and can be used with either the leaf shutter or electronic shutter, but they are much more expensive. As Godfrey alludes to above, it's possible to experiment with these older lenses without breaking the bank.
Hope this helps.
John