It is a very long process to clean the Aperture Oil in a Pentax Takumar 28mm 3.5 SMC but it is well worth it as this is a great little lens and are easy to find. Just make sure you have a spanner before you start repairing the lens.
This Pentax Super Takumar 105mm f/2.8 is a decent lens stopped down a wee bit and you can’t go wrong with it if you find a good deal. Being a M42 mount you can use this lens on some Digital camera bodies with an Ebay M42 Adapter. Also split focus screens from Ebay make focusing a lot easier!
The Pentax Super Takumar 50mm f/1.4 Asahi is another amazing example of a really well built M42 Manual Focus lens which Pentax is famous for. If you don’t mind manual focus then this little lens is great for even today’s digital cameras with M42 adapters.