A 300D turbo will be badged 300D. On the right side of the trunk, another badge will say 'turbo diesel'. A '300TD' is a wagon.

Lots of things to look for, and I'm no expert, but I'd say the biggest thing to look for is rust. In and around the wheel wells, around door trim, jack points, below the rear window, in the floor, the bottom of the trunk, and under the battery tray.

Nonworking AC is expensive, if you want it to work.

At 170K, it probably has not had a front end rebuild (lots of suspension and steering parts), but will need one around 200K, more or less. Having it done is very expensive, like $1200 or more. Doing it yourself is less so. Tons of people on this board are experts and have discussed the various pieces of this work at length.

Proper fluids and levels gives you an idea of whether the person selling the car has maintained it well.

Lots of things to check in terms of the engine; if you have no idea what to look for and can't find what you need here (hint: thread search), you really should just take it to a Mercedes shop for a pre-purchase inspection.

But you should really do some searching/reading. I'm certain people have posted entire prepurchase checklists here.