I will fly from Istanbul to Brussels for business meeting on a Saturday, but then need to go to Landsthul (near Ramstein)in Germany for a medical appointment on the following Monday. What kind of direct bus service do they have? Will it be better to return to Brussels for my return to Istanbul or should I try to leave from Frankfurt? What are the most economical options?
The easiest option is probably to take a train from Brussels to Frankfurt to Mannheim, then catch a local train to Landstuhl (only local trains stop here). And be aware, the hospital sits on top of a small mountain- you will probably want to take a cab!
You will definitely want to take a cab, not only is the hospital on a mountain, but it's at least 3-4 miles to the 1 gate that's open. There are several hotels in Landstuhl if you can't get into Billeting. I'd fly out of F-furt back to Istanbul, it's closer, probably cheaper and probably direct.