Yes, tiny quantities of old coolant won't hurt anything. But you should be able to get out 99% of the old junk with no problems. Don't forget to fully flush out the heater core, and open the drain on the engine block too.
For the thermostat, I'd recommend an OE dealer t-stat (not aftermarket) from FastLane, BuyMBParts, or your local stealer. I've heard some odd stories about the aftermarket ones, and possibly even the supposedly "OEM" stuff. Definitely don't use a McParts unit if at all possible. Oh, the wax pellet is sealed, so various contamanints (sp?) won't damage it.
