I'm not going to tell you whether you should worry about the thinner HG or not... I don't know what else was done. I'd ask the previous owner if he happens to have a list of all the specs from the rebuild. Mainly you want to know how much material was shaved off the head and how much off the block, then you want to know if it was bored over and the thickness of the head gasket.
What you're referring to is the SuperMonitor. It was an option for pre89 MKIIIs, not available for 89+. I don't know that I'd say they're super rare... They're not common, but they're easy enough to find. I've actually got two or three packed up somewhere. They were very cool for the time and they're an excellent reminder of how far ahead of the curve Toyota was back then.
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