Salt lake is a big town, you have to have a few old mercedes guys there. If no one here has a recommendation, go to the yellow pages or Google mercedes repair and find someone along 89. Call them and ask if they have success rebuilding 1960's 2.5s and what might it cost. I stopped at an autozone there once to get a MAF for my C320. I remember there were at least 2 mercedes shops on the way down from I80.
Go in and visit them You will see what kind of cars they are accustom to working on. Find out what machine shop they use and go visit that guy. I guess you need the valveguides and some new valves, so ask them how they put the guides in, (the old guys, like me, soak the guides in liquid nitrogen and heat the head.) If you just press them in, there is chance of deforming the guide. See if they can give a customer reference or a shop reference.
I'm guessing your best bet is to find a shop that was in business in the 70's & 80's And have them recommend a machine shop to do the work.
The issue is the pistons are typically not reusable and the tolerances are tight. So the shop has to take care at every step and not take any chances on short cuts.