For clawhammer you want a banjo which is more mellow than a bluegrass picker needs. Take one of your friends who play Scruggs style bluegrass - they will be able to determine if a banjo is playable. Basic points such as the height of strings above the frets, and a straight neck etc. are common for playability whatever style you use (a crap instrument is crap whatever style of music you play) If the "action" is too high you cannot fret a note without stretching the string out of tune, a warped, bowed or twisted neck will always make an instrument difficult (if not impossible) to play, uneven frets will either cause buzzes or possibly prevent a note being played. This applies whether you play bluegrass or old-time banjo (or guitar, or mandolin or lute). If a bluegrass player cannot get a tune out of the banjo, it is probably not playable.
Next step is to listen to the kind of sound it produces. A nice "plunky" sound will often suit clawhammer. Traditionally an open back banjo is more authentic, but other banjos will also suit. A "Whyte Lady" (misspelt) type of tone ring produces a good sound for frailing. Traditional designs with a proper "perch pole" construction, where the neck continues through the banjo head rather than metal rods holding the neck in place are more likely to suit the style.
As far as instruction is concerned, try to track down recordings of authentic players. Rounder records have some good stuff. I like Art Rosenbaum - "The art of the 5-string banjo" is an excellent record which inspired me. There is an instuctional book published by Mel Bay Publications which includes all the tunes on this LP and another of Art's (it includes them on a CD). I have just picked up a copy, but have been too busy to look at it in detail - 16 month old twins do not allow you much breathing space. The tunes are writtten in Tablature, which is worth taking the time to learn as it describes what your fingers do to produce the notes rather than simply giving you a melody or chords. I also picked up another booklet in the same series (again with a CD) of tunes played by the Fuzzy Mountain String Band.
Try this Blue clicky Thing Mel Bay Publications website I hope I got the Clicky right this time - They produce a lot of stuff which I do not like, but these two are definitely worth having.
When I started learningthe banjo (by the way I live in England, so was pretty far removed from any "source material", I could only find Pete Seeger's book, which scarcely mentioned frailing. It was however a useful starting point for getting the general idea of playing a banjo. The rhytmic pattern of Pete Seeger's "basic strum" are similar to clawhemmer, but in frailing you do not "pick" a string - every motion except the thumb is DOWN towards the banjo head. I could only reach a certain level of competence with Seeger's basic strum, as I found that the change from picking upwards to brushing down and then back to up prevented me from building up speed and momentum. When I discovered clawhammer, that obstacle was suddenly lifted and suddenly I could "fly".
That said - clawhammer can be hard, fast, driving music, but can also be slow, melodic and lyrical. I love the feel of it! It was clawhammer which inspired me to design and make my own banjo.
I hope my comments are of use. Feel free to "Mudcat Mail" me if I can help further.
Quack
Geoff the Duck