I'm not sure why 165 mm cranks would be better if you have bad knees - you're putting more pressure on your knee for an equivalent pedal stroke with shorter cranks. I have no cartilage left in my right knee to speak of and I prefer longer cranks, so YMMV.
As for buying them to take one apart - no point. There's very little going on there. I have no clue how old my cranks are but I'm guessing wicked old - the bike is a vintage 1988 Ti frame with some bizarre small BB size, so the square taper cranks I have in there now are going to be with me till I get rid of the bike. No modern parts will fit, and I periodically have the bearings in the BB rebuilt.
As for chain rings - if you wear them out they're easily swapped but they are not all compatible! Compact doubles have a different bolt diameter so don't be thinking you can stick a 34 tooth chain-ring on your standard crank, or vice versa.