Do a basic engine tune: check compression, valve timing/lift, PVC, exhaust, etc just like you should when buying a used vehicle. This will tell you the state that the engine is in. If its below acceptable standards, then think about the whole driveline, cooling/heating, etc., everything else associated to the engine. If the engine is in this condition, how well are the other systems.
You could do a quick rebuild with the engine block in the frame, but that may be only putting a bandaid on the problem and not a plausible solution. Without taking measurements of the internals when taking the block apart, you really don't get a 'rebuild' as you would think.
If anything, look for a used engine and check it out like above. It may be a lot cheaper and quicker to replace your old tired engine with something little better, then you can dig into your original engine on your time, money, and schedule.
Do note that life will change plans all the time!!