1994 Saturn SL2 speedometer and heating issue

Hey there. I have a 1994 Saturn SL2 Twin Cam with 190400 miles on it.

I recently had the alternator and serpentine belt replaced, but after the serpentine belt replacement, I noticed that my car starts to gradually overheat when it idles.

After the alternator replacement (which was a week ago), my speedometer has apparently bitten the bullet and won't function (complete with a Check Engine Soon light going on). The Check Engine light turns off whenever I come to a complete stop.

I can only hope that this won't put too big of a dent in my wallet. ?