It depends very much on what the OP is after. If you love taking pictures, well, I guess the FA the OP already owns could be a good compromise. Ideally I'd go for a Samyang 1.4/50mm and/or any f/1.2 50/55mm vintage lens, considering most people already have an AF zoom covering the same focal.
If you are interested in the history of camera optics and love vintage lenses, there are countless objectives in M42 or PK mount that have either a great build, an historical interest, or a peculiar optical signature. Sky's the limit
If you like both, like I do, the limit is eminently economical. We can't have everything!
To spend our money in a sensible way the first thing to do is to understand if a newer model is really better, IQ wise, or just adds the convenience of AF.
Excluding the FA Limited, all the others use the same optical construction generation after generation... and even the Limiteds are film-era lenses. Given the way the other Samyangs perform, I'd be curious to try the 50mm.
On average old designs are very sharp only at medium diaphragms, and vignette more than digital era lenses of the same focal.
If autofocus is required, the Pentax-FA 1.4/50mm is probably the best compromise.
It has AF, the optical layout is the same of the Pentax-M (one of the best normal lenses of its time), the price is affordable, and let's not forget that either in-camera or in PP it's possible to automatically correct its aberrations.
Availability of lens profiles can make the difference with old all-manual optics.
Cheers
Paolo