Welcome to PF Justin. Good discussion starter in your first thread.
I don't have a 135mm prime. But I did toy with the idea and got many wonderful responses:
https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/276646-how-...ime-aps-c.html
As for nifty 50s, I really like both my DA 50 f1.8 and my A 50 f1.7. The A is subject to flare and CA, but it is a thing of beauty and a joy to use. The aperture ring is useful for macro with extension tubes. The DA has the modern virtues: resistance to flare and CA, fast AF. And it is light weight and compact. The plastic construction doesn't matter. The A has the advantage of using 49mm filters, whereas the DA takes less common 52mm filters.
A-50 f1.7
DA 50 f1.8:
Either of these is outstanding value for money, and an excellent choice for a first prime.
But my new favourite "50" is the FA 43 f1.9 Limited. (Haven't got a sample worth posting yet, but getting there.)
Others will say the DA*55 or one of the older 55s.
They are all 50s really, in the sense that they are moderate tele on APS-C and normal lenses on FF.
@abmj makes a good point. The nifty 50 concept (that is, a 50mm prime as a single, light-weight, do-it-all normal lens) is really for film and full-frame. But I still find it a versatile and interesting focal length on APS-C.