Yep, that's a very good price for the 135mm.
The 75-150mm is longer and obviously a little heavier, but still takes 49mm filters. Image quality is probably about even.
London Camera Exchange have one on offer for £19.99 in their Worcester branch - if you want one I'd call them and ask if they'll post it. At that price you could almost buy both lenses for the price of the ebay 75-150mm!
Personally I'd say to go for both. Try them, if you don't like one then you should be able to sell it for what you paid for it.
If you want a bit more range at the expense of bulk and weight then the SMC Pentax-M 80-200mm f4.5 or SMC Pentax-A 70-210mm f4 would be perfect, they're both very sharp lenses with sweet handling. The 135mm still makes sense as it's much smaller and lighter - I use mine for urban photography as they'll tuck into a small Lowepro lens case which can hide on your belt under a jumper or coat. Stick a 35mm or 40mm prime on the camera and you're set, you can grab street scenes or isolate details to taste.
