The MX is a great camera. In terms of lens, I have the 50mm in F 2 and 1.8 in K mount Iand find them to very sharp, I also the 135 3.5 in M 42 and it also a great lens. I also like the 2.8 105 and the 28mm 3.5 which I think is better than the 2.8 all in M42. Some lens are rare like the 120 and 150mms. In your price range you can find 28mm 35mm 105 135 and sometimes a 200. Some of the Pentex zooms were also very good. I have had other 42mm lens such as Vivitar which I think were ok, but up to Pentax qauility. With an adpator you can work many excellent M 42s, including Fuji and Mamyia. Not sure what 3rd party lens are good in K.