As a non-pilot myself, I'd already worked out,
1- engine-life remaining
2- age is less relevant than condition
3- fuel-prices and availability are TOTALLY UNKNOWN for a decade hence
4. The value of a well-worn specialised-purpose heap of unwanted commercial kit,is probably salvage-value.
Given that you appear not to have worked through the above, I'm puzzled that you consider you are in a position to "pull together" a complete and realistic business plan.
You have already seen some tired old kit....base your costings on one of those,by extrapolating it's best economic service -life.
Photography is paid by results, not how flashy the kit appears!