You need to make absolutely sure there are no coolant leaks!! Is the header tank up to the correct level, once the engine is warm is the heater blowing hot air, after a run is the top hose solid? is there evidence of water blowing out of the header tank, did the temp gauge stay just below the middle?
Have a careful look at all visible hoses, make a very close inspection in the area of the thermostat housing and round the back of the water pump.
If possible, get a torch and mirror and look at the radiator - look for signs of leaking rad. If it has oil coolers, check around pipe unions. Get hold of the viscous fan (if fitted) check for play and also check that you can turn it relatively easily when the engine is cold. if it is very stiff when cold it is on its' way out or needs some TLC (clean and lubricate the bi-metallic spring and valve on the front. Has the fan nut been butchered with a hammer and chisel?
Take a DVM with you, check battery voltage and alternator output.
Look at condition of the coil and associated wires.
Check there is anti freeze in the coolant. Look at the block, check (as far as you can) that the core plugs are not leaking. If there is evidence of antifreeze on/around the bell housing/gearbox, the internal core plugs might be shot - walk away that's a drop geabox, engine out sort of job for core plugs that cost all of 30p.
Rocker cover gaskets are prone to weeping a bit. Make sure any evidence of oil is from the rocker covers and not the head gaskets.
Look around timing cover - look for any signs of oil or water leaks.
Listen to the engine, it should be smooth and relatively quiet. From idle, grab the throttle linkage and give it a yank to WOT, there should be no hesitation with revs rising quickly and smoothly then settleing down again to smooth idle.
As said above, check colour/condition of oil on dip stick and make sure it is up to the mark. Have a look at the oil filter - does it look old?
Underneath, look around the sump - the sump gasket does fall to bits and leak, well weep a bit (not serious or particularly difficult but a fiddle to sort out).
Look at any visible wiring - has it been bodged?
That should keep you busy, hope it helps.