Hi Mark,

overheating:
First you should find out the reason for the low coolant fluid level. Leaky hoses, a worn coolant pump or a blown cylinder head gasket might be the cause.
As Bastian said: Flood the thermostat cover bolts with WD40 or something like that for a long term, before trying to undo them. Thumping the bolt heads with a small hammer might also be helpful.

Due to the age of your car - irrespective of the low mileages - the timing belt should be renewed (if it isn't already done). In connection with the tooth belt renewing, replacing of all rollers, tensioner and coolant pump is even required.

Beste Grüße
Tim