If you have any doubts about the state of the cooling system I would completely drain the system, flush through with water under pressure, including the heater, with a hose pipe. I found the hose was a good fit to the heater pipe so I could blast it all through. Flush radiator, of course. I also used a flushing product, can't remember which, following the instructions before a refill with coolant.
The products aren't expensive, doesn't take too much time but does give peace of mind.