Hi and welcome!
It sounds a good car with all the major upgrades (although check the rear sprocket has been replaced with ether the MMC steel or Supermax).
Also check whether the S&S cambelt drive sprocket bolt mod has been done. This is about £100 for parts and maybe an hour of labour.
If it's still on the stock exhausts then either opening up a baffle or (probably better) changing to MMC Stage 1, Garage 56 or other more free flowing exhaust will transform the car.
Finally adding the MMC (or similar) rear mudguard is worthwhile.
Otherwise there shouldn't be anything major to budget for, just get out and drive.
Things will break or go wrong (fuel pumps seem to be popular at the moment!) but deal with these as you go.
In terms of an annual budget - I've averaged about £1,000 per annum for servicing (main dealer) and repairs over 5 years/20,000 miles.
