17, 20 and 22 are the most common heads found in the US. Each is an improvement over earlier heads so the higher the casting number the better though it doesn't seem that 17 heads are more problematic than 22s. Put another way, it doesn't seem that 22 heads are less likely to eventually need a new head gasket than 17s. Maybe you should pay more attention to head gasket selection than head selection.

The 17 head was original equipment on 90-91 350SD/Ls, 92-93 300SDs and 94-95 S350s. 20 and 22 were replacements. I don't know if factory crate engines came with later head designs. I wouldn't pay more money for anything newer than a 17 but I'd take the newer head everything else being equal.

There are 603 turbos in Europe. It's easy to tell one head from another with both sitting side-by-side. You pretty much have to go by the casting number. Note that some normally aspirated 603s have an SLS pump ahead of the crank sprocket. I've never seen a 603 turbo with such an SLS pump arrangement. That might help in your identification.

I would guess a head with valves but no cam is more than 20kg.

I'll have to search through the archives and GSXR's library for his instructions on trimming #14 prechambers to fit a #17 head.

Sixto
87 300D