The only issue with the M103 heads is the valve seals and guides and the head gasket.
If the fluids are changed as per spec with the recommended replacements the likely hood of need to repair is lessened.
Occasional use of the M103 engines leads to drying out of the rubber valve seals and guides and hence quicker replacement time.

Belt tensioners wear out. Periodic inspection should reveal when they are on the way out. A tensioner not installed correctly and adjusted as per the manual may lead to a short service life.

Duke2.6 has some very wise words in his post.