I’d start by finding the people who will maintain, insure and house the aircraft, then get quotes for that particular example.
For a D18S, the budget needs to account for both engines and propellers, their accessories, oil, scheduled maintenance and unexpected repairs. Review each engine and propeller separately, including calendar history and the scope of previous overhaul work.
Ask a shop familiar with the Beech 18 to identify upcoming inspections and applicable Airworthiness Directives. Recurring inspections can affect both cost and availability, so their next due dates belong in the buying decision.
Then check the practical arrangements:
- A suitable hangar, including door clearance and access.
- Insurance terms for the aircraft, intended use and named pilots.
- Any initial or recurrent training required by the insurer.
- Parts availability and the cost of getting the aircraft to specialist maintenance.
A useful request to the seller is an itemised maintenance history for the last few years, with unusual repairs identified separately. That helps distinguish routine spending from a particularly expensive year.
The Beechcraft D18S listed on CollectAirs is a starting point for that review.
Reference: FAA Airworthiness Directives.
Owners and maintainers: which expense would you put at the top of a buyer’s list?