Your temp gauge shoule be at or over 80 at normal operating conditions. If that's the only problem, you don't have a problem. My SD runs at 82 or so in the winter and 90+ in the summer - it's within the safe range.

I sympathise with you on your t-stat housing - I had screws break off as well. I drilled them out and used a nut and bolt all the way through to hold it on.

Don't worry about being a newbie - we all were at one time. I still am humbled by the lack of knowledge I have and the wealth that others on this forum have.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer