A good blister repair job if done properly is better than new. When new it may blister, but if the repair was well done it is unlikely to blister, at least for a long time.
As far as engine hours 1200 is not many. A marine diesel should last 10,000 hours if properly maintained. Maintenance is a bigger factor than hours and that is something to look into. Maintenance records? Best to pay a mechanic to check the engine out if you're not sure but if it runs fine it's nowhere near rebuild time, maybe 4000 to 5000 hours for a rebuild. And if not cruising full time you probably won't put more than 100 - 150 hours per year on it, probably less.
Living aboard in Victoria Harbour