Our company bought an Islander (300Hp model) from the Portugese AirForce that had been sold to them as a VIP transport for their Air Marshal (I would have loved to meet the salesman who sold it to them for that
).
It was in great condition though it had only flown 400hrs in 10 years (I wonder why ?).
Out came the toilet and all of the double glazing on the windows and it went on to start earning a living as a hard working geosurvey platform.
Within two hundred hours, we had to replace BOTH engines as there was major spalling on the lifters and camshafts along with glazed bores and frequent cracked cylinder rings.
The bigend bearings all had corrosion/spalling issues that ultimately kept clogging up the oil pressure bypass valves, hence the decision to rip 'em out and get on with our lives.
Once the control rod bearings had all been replaced, the aircraft went on to be as reliable as any Islander can be and ultimately ended up being a great machine.
So, the moral is...be prepared to spend money initially, but a low time old aircraft CAN be a good buy if you get it for the right price.