The question back to you is whether you're trying to avoid cost or work. If I could trust someone to do a good job for $80, I've done enough tranny servicing that I don't care to go back down there. But it's a chance to check for leaks, condition of shift rod bushings, flex disks, etc.

Dexron II or III is good. It's hard to tell what you're getting at parts stores these days since no one uses Dexron III in labels anymore. Even Mobil 1 no longer sells ATF with a distinct claim to be Dex III compatible. There's a synthetic version of Dex II available from Redline or one of the other reputable synthetic oil brewers. The really meticulous MB owners I know use synthetic Dex II.

Mind how you fill the tranny. Initial fill is only what you drained from the pan and maybe a quart more. Fill the rest a little at a time with the engine running checking the dipstick frequently until the fluid is about 10mm (each application has a specific gap) below the cold mark. Get the transmission to operating temperature and top off as necessary.

Sixto
87 300D