From reading your post, you are looking for a 3.2 carrera. Okay, but please do extent your search for the 3.0 SC cabrio's as well- they were only made for a year but pose a fantastic alternative to the 3.2 and are a wee bit lighter.
There is also a dedicated forum on the internet on these "impact bumper" porsches- google it and it will direct you. I can certainly advise you to purchase Pter Morgans "the original porsche 911" and Paul Frere's "porsche 911 story" for the stats, details and mechanical facts.
1. The cabrio with the 915 gearbox weighs 1210 kgs vs 1160 kg for the coupe. The 915 type gearbox was offered until July 1986 after which the g50 gearbox was introduced with the H programme (model year 1987), effective from august 1986 until july 1987. Porsche model years run from ugust until july. The G50 gearbox cars weigh 1260 kg (cabrio) vs 1210 kgs (coupe)
2. mechanically there are no differences but there are country specific differences : Swiss and Japanese market cars/ European market and German market cars were catalysers were introduced a lot earlie / US market cars/ UK specific cars RHD (country code C16) vs Singaporian/ HK RHD cars etc
3. Values: typically 10 % more for cabrio's than the coupe, targa typically 10% less than the coupe. But this is the rule of thumb for the UK; on the continent open cars (cabrio & targa's ) can often fetch more money than coupe's in sunny dry weather markets. Its best to go for condition rather than colour or type etc as a bad one will cost a lot more than buying a good one in the first place.
4. Cabrio's do have less regidity than a coupe without sunroof but having said that: they are build like the firth of forth; very very solid indeed if they are not rustbuckets. For normal road use and the occasional trackday the difference is marginal and your preference is more important than the rigidity of the car. The coupe however has the iconic shape and was available in the UK since the beginning: targa's for example were only offered in RHD form from M/Y 1972 onwards so for a lot of porschista's a coupe is the only one true 911.
HTH,
Bert
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