Behr radiator or Nissens are $150-180. remove clips holding radiator in place, loosen transmission cooler lines, install radiator and fill with coolant.no need for a Mercedes radiator. This you SHOULD do yourself at least and save some money..you have a couple of thousand posts here so i am sure you know your way around these cars
Headgasket: 800 for parts including a graf water pump. labor around $1000ish; head resurface and machine shop is about 200-300 ... 2500 tops. No need for valve seals and guides on the 104 engine unless overheated and neglected for a long time.....mine did not need anything at 180,000miles !!!!
AC repair: I am not very experienced in that area but you could put some dye in it yourself. if no visible leaks then most likely evaporator core inside dash....easy job, except dash removal is the big deal....i think it is a 10-12 hour job at a Mercedes shop. This in fact is the biggest and most expensive job on these cars (aside from transmission rebuild)
whatever you do, make sure you take it to a Mercedes specialized shop. regular mechanics will have a hard time getting timing correct and sealing front cover correctly...
edit: AC.....Not sure how to diagnose leak unless engine is running. but most likly it is the evaporator...it is not a matter of if - but when they fail...faulty design
good luck