There should be no reason for it to fail. Provided all the maintenance is kept up to date, regualr oil and filter changes, then it should go on and on. The reason why the engine would fail would be because of inadequate lubrication to critical componets. The engine is designed to be in perfect harmony and therefore should have no reason to fail. However, swarf in the oil can cause premature wear the crank journals, once that happens it won't be long before then engine goes.
Those cars don't cost the earth, you won't see much gains from just selling it. Better to drive it until it breaks. It's unlikely to just fail with no warning. The problem occurs when it comes to consumerables, clutches, tyres, batteries etc...and then you have to say is it worth while. Even if the engine develops, knock, wear, etc...a full engine rebuild will cost less than you think. If you delivered the engine to a specialist, you're talking around £600 for the parts + labour. Well at least that is what it cost my brother to have his GT engine done, looking at the paperwork anyway. God knows what tha means. Drive it till it goes, then decide what you want to do. Point being is engine failure, it's one of those things. And if you're smart, it'll just be minimal work. Crank reground, new bearings, seals, pistons and rings. Plus the gaskets. All of which genuine doesn't cost that much. Then, you have a car with ZERO mileage and you know as a fact will last you a lifetime. I'd say the only thing that really bothers me is rust. The cancer of cars once it starts it difficult to stop and needs to be cut out. If the shell is mint then happy days, there is not a lot that can't be fixed.