if you start skimming/porting etc the head your gonna go well over budget! id just clean it up and whack it on. whats the mileage on the head?
Home porting won't cost much and you have tools you can use on other jobs, so they shouldn't be counted against the budget.
It
will should ensure the man makes his target BHP, too.
A skim is precautionary and should ensure the new headgasket seals. The clean is necessary to get the swarf out from the skim and the home porting waste, too.
While it's there having work done, why wouldn't you get the stem seals replaced? Who says the seller either knows or is genuine about the mileage the head's done.
As I said previously, it's de-risking the job - spend £50 now to avoid possibly spending another £100 in a short while.
A £100 budget for anything where the head is coming off an engine is rather optimistic, just based on parts alone. Is it the budget that is crucial here, or the output of the motor?