Congratulations!! Welcome to the 5-cylinder world!! Where are the pictures?
If you don't know when the last valve adjustment was, doing one is a really good idea. These valves tend to wear tight, so the engine will act like the cylinders have low compression.
Regarding the Diesel Purge...yes, doing a purge is a good idea. However, I don't think I would buy that kit...I haven't run any numbers, but I think that the individual parts can be gotten cheaper than the kit. Hoses are cheap, and any glass (or probably even plastic) container will work for holding the liquid. I have yet to do it myself, but based on what others have said on forums I used to frequent, the stuff you'll see come out of the return line can be NASTY...if it's bad enough, I would do it a second time and possibly replace the filters twice. Just, make sure you use the actual Lubro-Mahle Diesel Purge and not something else...and, a pure treatment (using the DP as a sole fuel source, as described on DieselGiant's page) is really the best way to clean things. Just pouring the stuff in the tank dilutes it too much for it to really do any good.
One thing to keep in mind with M-B Bosch injectors is that they're not throw-aways (like the Stanadyne injectors used on the 6.9l/7.3l IDI). So, keeping them clean is a good thing...they can even be disassembled to be cleaned, if you have the time and inclination. There's a write-up on how to do it somewhere online...I MIGHT have a copy sitting around, but I'll need to look for it.
What's the front suspension like? My old '85 300D was in good shape when I got it, except the front suspension and the interior...and, on my car, the front suspension went from bad to worse, and that coupled with some body damage from the ex-g/f is why I ended up giving up on the car...