An Apple Retail Store employee in the USA advised me that I should not purchase Apple Care Protection Plan in the USA for a laptop I had purchased there but which I would mainly be using in the UK.
However, I believe you are entitled to buy Apple Care Protection plan in your home country for a machine purchased anywhere, and it will be valid in your home country, but I am still not 100% sure on this... it may only be true for portable products and not desktops, as Apple couch AppleCare as being an 'extension' to your standard warranty.
Regarding customs, I can only speak from my own knowledge which is regarding importing such an item into the UK, although I expect it is similar for people based in other countries, especially Commonwealth countries like Australia.
If I buy a Mac in the USA, then theoretically when I enter the UK I am obliged to pay:
- Import Tax of 3.5%
- Value Added Tax (VAT) of 17.5% on the new total
- An administration fee levied by customs and excise for their role in collecting the tax
Obviously, if one manages to get through customs without them being aware that you have a nice new computer in your suitcase, then you are laughing all the way to the bank, although some highly moral people might suggest you are defrauding honest taxpayers from their rightful share of the tax you should have paid... but that argument has never convinced me