Pretty much any Triumph or MG is a decent choice as there are plenty of companies providing the parts, and plenty of clubs providing the knowledge. The club triumph forums are a mine of knowledge and, though not as easy to use, the Triumph Sports Six Club site is useful too.
eBay is always chock full of Spit bits making the restoration a bit more economical than some other models and marques.
Somebody once told me "buy the best bodywork and don't worry so much about the mechanics" which is decent advice. The less welding/bodywork you have to do the happier you will be and the less daunting the whole experience will be. Mechanicals are (relatively) much easier with only some parts requiring more specialist knowledge (like gearboxes, overdrives, some parts of the engine rebuild like boring, skiming, etc.) that you will probably have to pay for if you need it.
The spit is also nice as it's a chassis car, which means that you can whip the whole body off (he says, easier said than done
) and have a good look at the chassis, fixing any issues and making it easy to rustproof. You can also spray the body tub underneath in one go that way too, making a lot of the "awkward to get at" parts of repair/rustproofing more amenable.
The wiring is simple, the brakes purely mechanical, the suspension simple (although it can be difficult to set the geometry up if you haven't got the knowledge/tools/etc.) and so on. The interior is quite cheap to re-trim and there is a lot of tuning/customising support out there. For example, fitting MX-5 seats is a common job, most cars have tubular manifolds and sports exhausts, and so on.
Out of all the cars out there, the Spitfire has got to be very near the top of the "easy to restore" list.
