Personally I'd have the MX serviced/CLA'd. Unless you feel comfortable replacing the light seals and giving it a good clean yourself. Look up Asahi Photo or Harrow Technical.
Any MX you buy will need new light seals and mirror bumper foam unless they've already been replaced. It's not hard to do though. Look for pre-cut kits on ebay, they usually come with instructions. The grubby focussing screen can be replaced very easily, there was a range of alternative screens made for this camera. Look up the Dimitrov K-Mount Site for more information. Note that while screens made for the LX will physically fit into the camera they will throw the metering out.
As for lenses you won't find the ones you mention for £35 each. If money is that tight I'd suggest you:
Keep the Pentax 50mm f2, it's a decent lens when stopped down a bit and perfectly capable of top quality images. If you see the 50mm f1.7 or f1.4 (or the 55mm f1.8 or f2) then go for it, but don't worry too much about replacing this one.
The SMC Pentax or SMC Pentax-M 28mm f3.5 is brilliant and you might find one that cheaply if you keep your eyes open.
Telephoto - you can often find a Pentax-M 135mm f3.5 within budget, it's tiny, sharp, has a built-in hood. I prefer my SMC P 120mm f2.8 but you definitely won't find one of them for £35 (I paid £65 and have only seen about three for sale in the four or five years since)!
There is also the Pentax-M 80-200mm f4.5 or the Pentax-A 70-210mm f4, bigger and slower but if you want a zoom they're decent performers. Also look at the Pentax-M 75-150mm f4 which is a gorgeous little zoom and can often be found within budget.
As for third party lenses the Vivitar 75-205mm f3.8 is very good, can often be found cheaply.
Last edited by PentaxBronica; 01-23-2013 at 02:13 PM. Click to view previous post history.
Reason: Focussing screen data