Thanks to everyone for the advice!
Summarising what I need to do:
1) Estimate how much the disassembly will cost
2) Select the parts that we want to keep
3) Get a quote for the price of these parts as if we would buy them from a vendor
4) Get a quote for the remaining parts and the scrap metal as if we wanted to sell them.
5) If (aircraft purchase price + cost of disasembly+cost of overhaul of parts-revenue from sold parts) < purchase price of the parts we want to keep, then we should go ahead.
I might also factor in the financing costs, costs of ownership of the parts, some parts being damaged beyond repair during the disassembly, etc.
Is this correct?