Welcome aboard to the shop forum! I have a 1985 300D with 270K on it and it runs great (knock on wood).
I think that one of the most important things to look for is how well maintained was it by the previous owner, i.e. maintenance records, frequency of oil changes, etc. Pay attention to how smooth the tranny shifts. Check for leaks. If possible, check how well it starts on cold mornings (kinda' hard to do this time of the year). Does it have the block heater installed? If it has a sunroof, does it open and close smooth? How does the central locking system work? What about the operation of the power windows (if so equipped)? Look under all the floormats for rust (a sign that water has been leaking into the footwell areas).
I'm sure alot of the other members can give you additional ideas of things to look for.
$6grand does seem a bit high in my opinion.
84 500SEL EURO 87K
85 300D 267K (and still goin' strong)