1997 Villager with 155,000 miles, with original rad and core heater.

Had coolant leak with steering seizure, so brought it for serveice and they replaced water pump and tensioner belts which drives the steering. A week later and I get low heating coming into car. I brought it back and they said it was they suspected a plugged core heater and did a back-flush. It worked ok for a while but now it's just cold air coming from car. I also notice a little coolant loss, which I top off once in a while. Today I noticed engine overheating, not quite at the danger mark but about 4/5 of the way there, and the coolant level is a bit low in the reseve tank but no major leaks. Any suggestions ? I hope it's not the head gasket (Is there a way to check this)

There is a few ways to check if the coolant is leaking due to a head gasket. 1 and the best is to preasurize the cooling system and see if it maintain's the pressure. If it does not hold there is an almost certain chance the head gasket is gone. Another way much less scientific is to smell the exhaust. Don't inhale it now you don't want all the Carbon Monoxide but just sniff it and if it smell's sweet then the coolant is most likely leaking into the combustion chamber. Also look and see if your oil looks like mud. If it does then coolent is leaking into the oil chanell's.