I've tried a number of
preset t-mount tp's and so far the 500mm f'8's have not impressed me. Although it is on first sight just an incremental step up from a 400 6.3, I find the latter a big step easier to work with, and to produce consistently better pics. One of the big problems with the 500's is the long minimum focus distance.
More info here (see the video review by Oneil, link in description, of a very similar lens.).
Pre-set (Quantaray, Kimunor, Five Star, etc.) 500mm F8 Lens Reviews - Miscellaneous Lenses - Pentax Lens Review Database
PS pick up s/h one off ebay for less than $50 if you want to check out - i wouldn't pay $120.