If you're losing coolant, first thing is to find the leak. A mechanic would do a cooling system pressure test - this should quickly locate the problem. When you start the car after it's been sitting overnight, does it run on 3 cylinders for a few secs? If so, there is possibly coolant leaking into a cylinder.
A compression test might tell you if you have a hg leak and where, but it's likely to be inconclusive. A cylinder leakdown test is a better option.
If it is a head gasket then luckily it's very straightforward on a sud/33, and no problem at all with the engine in the car. Probably take little more than an hour to remove one head, and maybe 2 hrs to refit. The head gaskets themselves are also cheap.