Thanks, although the problems with KEH, BHPhoto and Adorama is that the postage is often far too expensive to justify buying the equipment from the UK so for now I think I'll just stick with eBay.
Thanks again everyone for all the advice, very helpful! I think I will try to get these lenses: SMC 28mm f/3.5, (maybe a 35mm f/3.5 but they seem a bit more expensive), SMC 50mm f/1.7 (or f/1.4 if I find one cheap), and an SMC 135mm f/3.5, or if I can find one an SMC 120mm f/2.8. Does that sound good for me? I think I'll also have a look at some reviews etc. of the M42 mount pentax lenses, as a few of them seem quite good.
Also I found a black MX with SMC 50mm f/1.7 for £65, it looks in good condition with only a bit of paint loss, especially compared to my MX and has new light seals, is that a good price I should think about? Or shall I wait and see if I can get anything cheaper?