when having cylinder head work done, should both sides be done at the same time or just the side with a bad valve?

If you've gone to all the trouble to remove the head, get everything done. You would kick yourself in the butt if you tried to cut corners on a job like that if something else went south. The biggest part of the job is taking the engine apart (Just ask me :D :D :D) If you've gotten this far, don't short yourself. Remember, to pay for head removal once and fix everything is still less expensive than head removal twice and fix the stuff you skipped the first time around. There are circumstances where it makes sense to cut corners, however, this ain't one of them. "Measure twice, cut once", (ancient THIS OLD HOUSE PROVERB)

2400 dollars to have both cylinder heads done. Labor and parts, thanks or the advice. Car should be ready next week. I look forward to many happy miles. Anything else to look for while engine is apart?

I'm sure that whoever is doing the work for you will be looking for broken stuff.(they always seem to find stuff that needs replacing or repair). Cams, followers, lifters, condition of the pistons, the deck on each side, You could look forever and probably find something...

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Jeff L
1987 300e
1989 300e