Definitely have an HPI check, as you need to know its not been crashed and that there is no outstanding finance on the car. Try
www.hpicheck.com
Ideally, prior to the final purchase, you'd have the car inspected by a local Maserati specialist or main dealer, at a cost of approximately £200 plus VAT. But if you're not doing this, make the standard test-drive basic checks such as starting the car from cold and checking it sounds okay, look for any oil leaks, check all the electrical goodies work and that there are no warning lights on, and check that it drives and brakes in a straight line with your hands of the wheel.
Check the paperwork, the V5, the book-pack, service history. And makes sure the chassis numbers on that match those on the car.
And finally, if buying privately, do check that the person you pay is actually the owner!
For insurance, you might be best to call a specialist broker such as Pace Ward Insurance (01782 286311). But no harm looking on-line also.
Usual common sense stuff really.