Points to consider:

- the tensioner has to come out. It's a ratcheting tensioner that must be reset before installation.

- the distributor has to come off to replace the cam so have a horseshoe gasket ready and read up on the sealant recommendations for this critical sealing point. There is a different sealant spec for the corner where the head meets the block and the gasket contact surfaces. I used Toyota brand semi-drying sealant (for oil pans) in both locations and had no leaks.

- there was an oil feed line redesign. The manual says to use the later design. I don't know part numbers but I believe this was settled in the early days of the M103 so a 300SE/L (88-91) should have the new design.

- early hardened camshafts should be installed with matching hardened rockers, later chilled cast camshafts should be installed with matching carbide metal coated rockers. The break point is early 1989. I don't know how to tell one type of camshaft from the other.

- how new/old are the valve stem seals?

$650 for an M103 is not a bad price but is it better than what you have? It probably needs head work. Since the bottom end is pretty tough, you might be better served by getting head work done to your engine.

Sixto
95 S420
87 300SDL