Porsche has a reputation for testing cars in extreme climates in preproduction. Additionally, they have a lot of experience with racing in desert environments. Based on this, I don’t think that you should have a problem with the 911 in extreme hot weather. This is especially true, I would imagine, for the water cooled models you indicated you where interested in.
As far as which variation i.e. C2, C4, convertible. That is a personal preference issue. People who typically select the C4 do so because of the safety and traction advantages especially required in climates that have rain or snow. From what I know of Dubai that probably isn’t as much of a concern for your situation.
You still may enjoy a C4 for the added stability the 4 wheel drive gives at the limit. The C4 costs a little more and has more mechanical parts requiring potential repair and maintenance issues. Additionally, some indicate that there is more understeer associated with the C4 vs C2 and as a result prefer the sharper handling and lighter weight of the C2.
As far as a convertible, the soft top has les structural rigidity which affects handling somewhat and could result in squeaks and rattles as the car ages. Although Porsches are better than most in this regard. So if you like convertibles I wouldn’t be afraid to get a Porsche Cabriolet.
Overall if your interested in a sports car you can hardly go wrong with a 911
.