If theres a used plane on market that fits the bill but has a poor condition despite C and D checks being a way off, is there a way to help improve its condition if/once bought?
Yes. Do more checks to it or visit the maintenance page for the plane and do extra work to improve the condition.
Personally, I would just push it up to about 75-80% and put it in service, as regular maintenance will bring it up over time from there.