Single point can be prone to the odd back fire, but it's almost always an indication that something is amiss in the engine and/or management system. Not, I would say, something to put you off.
SWMBO has single point fitted to her Disco and it has run faultlessly until, say, the valley gasket failed.
Multipoint (or 'gas phased injection'), when set up correctly is awesome, but I've met very few who can do it properly. And yes, it should be twice the price because the control systems are much more complex (it'll have its own management system) and there's a lot more plumbing involved. When they go wrong - they are also very expensive (800 squid for the ECU?). They can provide superior performance to the single point system, but not sufficient in my book to justify the extra cost (in most cases). Highly tuned engines would benefit from it but otherwise.......
1200 squid for a fitted single point system sounds about right - just make sure they can supply all the right certificates. Oh, make sure you have at least 70ltrs capacity (35 in each tank).
Underbody gas tanks present very few problems and are stronger than the floor they will be hiding under. They will deform the floor long before you put a dent in 'em! Hence a friend (with multipoint on a 300hp Rangie classic) has two mounds by his back doors from the gas tanks!
Hope this is useful
Cheers
8)
Eeyore