my thoughts;
You can a buy a fortigate anywhere and uses it. It' s probably cheaper due to American-taxes vrs VAT charges in the EU area in general.
E.g VAT in UK can be 15-18% or more , where as sales taxes in Florida/USA is aprox 8.5% depending on county location
For support contract, your best is to buy a support contract from a vendor in that country that you plan on operating. I had problems where I had to ship an appliance at my own cost back to a USA vendor in order to get support.
You will also need to check if you would have to pay VAT upon arrive. Example I order something in the USA and shipped it via DHL to London and had to pay VAT on top of the USA sale-tax. So I got hit twice with added taxes that I didn' t take into consideration.
So be careful and check all of avenues b4 buying. I would look at Amazon or Ebay and a UK dealer if your in the UK and just buy it. Unless your taveling to the USA o vacation and business, than pick up a few stuff them in your bags

I personally don' t think there' ' s any shortcut with eliminating value-add-taxes completely. You will pay the systems at one of the pond in VAT, Shipping or other costs.
Do your math and planning.