I've built the AMT's Viper, and it's one of their descent kits. Both the Diablo and XJ220 kits have some flaws, but I've seen some nice built-ups, so they can be built into nice replicas.
I believe both the 911 & Countach kits are tooled by the Italian company called Italeri, and their kits are just about average and fairly easy to build. Their weakness is always tires.
I think either the 3 kit or 2 kit combos would suit your need. You probably want to build more than one to improve your skills, so those sets offer a great value for money.
BTW, Rainbow 10 takes paypal, so ordering them is very easy; just so you know.