I spent a lot of time wondering what lenses would make a complete setup for me. So over the years I have collected the following lenses for my two Pentax KX bodies:
Pentax SMC 28mm f/3.5
Pentax SMC 35mm f/2
Pentax SMC 50mm f/1.4
Pentax SMC 55mm f/2
Pentax SMC 100mm f/2.8
All of the lenses perform absolutely mindbendingly well. But I only ever use the 35mm and the 50mm. It's very rare that I use any of the other ones, as excellent and useful as they are.
The 55mm f/2 lens is a really amazing performer, and great for portraits. Really good wide open, and mindboggling at f/2.8.
The 28mm f/3.5 is the best wide angle lens I've ever used in terms of sheer picture quality.
Those are both fairly inexpensive and wonderful lenses to own; good substitutes for other more expensive lenses.
Regarding the 135mm f/3.5 - it was one of those lenses I owned for a decade and used maybe five times. I gave it to a friend who got a Pentax K1000, and she has used it a handful times in the last two years. I guess it depends on what you want to do with it. I found it a bit too long for portraiture, and a bit too short for long distance shooting. For street photography it's tough because of its max f/3.5 aperture, and you need something like a 1/250th second shutter time to get a decently sharp picture. So, it's one of those focal lengths that have proven less than useful for me. But you may have a need for a medium telephoto lens that I don't anticipate.
Also I found an MX for £120, it comes with a 50mm f/2, strap, new batteries and has been fully serviced with a year guarantee, is it worth it?
about $190 US, I see a few MXs in C condition without a lens running for $180 or so, seems to be a fair price.
Okay I will really think about buying it, although there is one bidding in similar condition (although not serviced so may need new mirror foam or something) with a SMC 50mm f/1.7 that started at 99p and is now only on £1.04 so I'll have to keep an eye on it!