The engine in the 150 gasser is a Continental 3 cyl if it's a 1964-68 model. It's a Perkins gasser from 1969-74.
I check availability of parts before buying a 40+ year old tractor. The engine rebuild kit is available
Massey Ferguson Engine Overhaul Kits
In Jul06 I bought a 1964 MF-135 diesel with Multipower for $3600 (no FEL). It had sheet metal damage, but the engine runs OK. Has a few oil leaks now that I need to fix.
The average trade-in value (what a dealer would give) of that 150G runs from $1300 (1964 model) to $1900 (1974 model). The average resale value (what a dealer gets on resale) ranges from $2600 to $4000.