There was an old rule of thumb that an aircraft will cost its purchase price again over the next three years to keep it going.
The suggested bottom up process sounds good.
Ins£1000
Hangarage £4000
Annual £3000
Broken Bits on the way..£2000.
Looks like the rule is fairly valid. Good luck with your purchase.
I've always though it is much easier to buy into a group that has already gone through the setup and their hourly/ monthlies will be realistic and proven.