Just picked up a 1991 240 sedan 135,000 miles. The car has a totally unknown maintenance history and no records, for the price I was willing to take a chance, its in great cosmetic condition. I bought it in New York and drove it back to Chicago, 700 miles or so. I was pretty amazed at the way it ran for the whole trip, although the sunroof leaked like a sieve after taking it through a high pressure car wash. The main problem I noted was that the car never warmed up to "normal operating temperature the gauge read 1/4 of the way up at the highest for the whole trip I was assuming it was the gauge itself as the heater seemed to have decent, but not great output and the car seemed to run ok. About halfway through the trip the check engine light came on, everything seemed to be fine so I drove on, the car didn't seem to burn/leak any oil, got about 25 mpg at 70 mph.
When I got back I took it right to my mechanic (who is not a Volvo guy more a Mercedes BMW Guy) for an oil change and an inspection. He said it was in good shape for the miles except for the Main engine seal was leaking pretty bad he suggested replacement, he also said the 02 sensor was bad hence the idiot light. I was planning to do the timing belt anyway so it makes sense to do it then. I asked him about the temp gauge issue and he ran the car tested with a temp gauge and found the engine was indeed NOT heating up to factory specs, he thought it was probably a thermostat stuck in the full open position. I set up an appointment for the timing chain main seal for next week. In the mean time I decided to do what I could to do some DIY catch-up maintenance, I replaced the fuel filter(really bad) transmission filter/gasket, flushed the coolant and replaced the thermostat. The thermostat replacement seemed to do the trick, came up to a little less than halfway on the gauge and seems like its staying there. The thermostat I put in was a non OEM rated for a 180 deg temp as opposed to factory spec 190, Is this a problem? Did I do damage By driving the car so long with a stuck open thermostat? What problems can be caused by under heating? I also understand from reading this site that the flame trap being clogged can cause heavy backpressure and blow out the main seal.(my German Auto mechanic didn't mention anything)Is it possible that if I replace this flame trap that this will alleviate the main seal issue and save me an expensive repair or is it to late once its leaking?

Post turning more into a tech qustion than an introduction so I'll stop for now. This site is amazing, Thanks for the great info so far.