Car is a 1989 245DL automatic

I don't expect to hear all good news, but just some info to give me a peace of mind. Here's the scoop. When I bought the car I had noticed that the temperature needle was a little below the 9 o' clock (middle) position, sometimes lower. One time it wasn't going up at all. But normally the temperature never rests on or above the middle position; it's usually a quarter up from the bottom. I didn't think much of that because I thought that may be a good thing, since I and many people dread an overheated engine. The car runs great, and I don't notice anything unusual. It's just recently I became concern about running a cold engine after doing a search on "thermostat" here. I think my thermostat may be stuck open. The low temperature reading kind of confirms this. And after driving for a while (30 minutes or more), I also noticed that the engine doesn't feel "normal" hot. I mean it is hot, but not hot hot, like in most cars after driving even for 10 minutes.

Now, I read my Chilton's manual (ugh, that book covers too many models and model years and is too broad) and it said that running a cold engine is "rarely disabling"; however, it may prevent the oil from performing optimaly and may cause poor fuel efficiency and in turn poor running. Otherwise, it did not say that running a cold engine will permentantly damage anything. I conferred with someone on the internet, and he also said running a cold engine will not do anything bad or cause any permentant damage.

On the other hand, I remember reading a post in the archives that said running a cold engine may do something to the cylinders or something inside the engine, something about deforming parts or something to do with engine pressure.

The car runs fine despite a low temperature, assuming it is a faulty thermostat causing it. Here is the big question: What are the consequences (short-term or long-term) of running a cold engine or having an open thermostat? I've driven the car in this condition for about 125 miles. SHOULD I BE CONCERNED?

I will not drive my car until I get a new thermostat, probably not until the middle of the week. How much will a shop usually charge for changing the thermostat and do they have to flush the coolant? My cooling system is fine. I hope changing the thermostat does not entail emptying the coolant to access the old thermostat! Thanks guys!