Hi Raider,
As far as premium lenses go, I'd suggest a Canon FD 300mm f4 L as one of the best "bang for buck" 300mm primes. An FD2Q adapter can be gotten cheap, and it's possible to find a decent copy of the lens between $300-400 USD. I have one that was converted to K mount by a user. It was in "beater" cosmetic condition, as a previous owner had more than prominently scratched its initials on both the barrel and integrated hood, probably with a dull axe. . .I covered these with electrician's tape, and it actually looks good from a distance. . . in any case, I paid $225 for it IIRC, and have been impressed with the lens' performance. I have 2 FA* 300/4.5s and 1 each Tamron SP 300 f2.8 Adaptall 2, Sigma EX 300 f2.8 APO, and an FA* 300 f2.8, so I am familiar with most of the top end 300mm lenses in K mount. I find the FD 300/4L (maybe not so surprisingly) in the same league optically with the newer premium AF optics. It's sharp wide open (which you want with the Q) and MF is fine and precise. It controls CA/PF about as well as my FA* 300/4.5s. This is better optical performance than you will find in the K, M*, and A* versions of the 300/4 from Pentax at about the same price point or less. Some samples at
https://www.pentaxforums.com/forums/136-pentax-q/180716-my-new-long-lens-q.html#post1887982
An alternative, that might be more useful since you have Pentax DSLRs, is any of the DA 55-300 variants. For me, this is possibly the best compromise between optical performance and practicality (carry weight/size and convenience). The zoom capability makes spotting a lot easier than any of the 300mm primes. Optically, it's not up to the same standards as any of the premium primes, but it's not really that far behind. It's usable at both wide open and stopped down to f8 without losing too much to diffraction (which, in reality, isn't nearly as much of a problem as some may postulate). I have found this combo usable handheld with reasonable technique, using a magnifying loupe only to shoot sparrow sized subjects at 10-15 ft and diving birds like grebes and mergansers at much farther distances. Some examples at
https://www.pentaxforums.com/forums/136-pentax-q/204091-q-fw-v1-1-da-55-300-h...ml#post2156152 and
https://www.pentaxforums.com/forums/136-pentax-q/223850-q-da-55-300-pied-bill...ml#post2374943.
Personally, I own both an original DA 55-300 (with the metal mount) and a WR version, but the original is the one I usually use with the Q as it's zoom ring is considerably freer to rotate, so it's faster to use. I wouldn't shy away from the DA L version for use with the Q, but I'd get one of the aftermarket hoods for it since it does not come supplied with one.
Scott