Have a look at www.importcarparts.co.uk for bolt and gasket prices, theres many to choose from depending so that will give you an idea,
I'd say a day to strip down and rebuild all being well also depends if theyremove the motor, (ideal world where your mechanic can just work on yours) if I'm wrong I'm sure somome will correct me,
Head skimming cost me £ 25.00 last time I had one done, not sure about this but you may not be able to have the head skimmed again. And you'll have to taKe into account theres a pair of heads on a subaru so the other side may need to be done as not to effect the compression, again I may be wrong.
I know of a forester being done at the mo, he's not expecting much change from £900