Here's my opinion of each of the cars you mention.
But before you buy ANY car, take your final choice to a local dealership and get a FULL inspection of the vehicle - have them go over the car from nose to tail with a magnifying glass and give you a report of everything they see that is wrong with it. It should cost you about $150 but it is well worth that money to not buy the wrong car.
Also, before you buy a car, find out whether it has a timing belt and, if so, how much does THE DEALER (don't go to independents for a timing belt change) charge to change it.
You also want the seller to show you proof of the last change and, if they can't, deduct the cost of that service from the price you're willing to pay for the car.
1. Boring but solid choice. Fine as a "city car" (i.e. short distances at less than freeway speeds) but rubbish for long drives.
2. Another decent choice. Probably better for long Don't listen to people whining about American car reliability. Like any other car companies in the world, some models are better than others. If the Cobalt was a bad car, rental car companies wouldn't use so many of them.
3. I've never owned a Volvo but I would be worried about the cost of replacement parts. I don't know if it is true but I've heard people say that Volvo cars don't often go wrong but when they do the cost of buying replacement parts is quite high. Still, the 2006 S40 is from the time when Ford owned Volvo and it based on the Ford Focus platform. If they used lots of Ford parts then they shouldn't be too expensive.
4. There's an old saying that there's no car as expensive as a cheap Ferrari. In the real world, the closest regular people are likely to get is a cheap Mercedes. I see cheap Mercs everywhere. I used to own one. One of the most expensive cars I've ever had. I paid $10k for a beautiful 450SL convertible and then spend almost half that in a year keeping the damn thing on the road. I let it go when it developed a problem with the engine that turned out to require a replacement. I sold it to a mechanic friend who was willing and able to undertake the job.
5. Anything built by Kia or Hyundai in the last ten years is probably a safe bet in terms of reliability and fuel economy.
6. Probably would be more comfortable for long drives than anything you've listed so far - if that's a concern for you. But if that is a big deal, I would go for what's next.
7. If you want reliability and cheap repairs... you can't go wrong with a Continental. It is based on the Ford Crown Victoria and there's a reason why those cars were the nearly unanimous choice of cops and cabbies over pretty much its entire history. Simple design and bulletproof reliability. I used to work with a guy who had 400,000 miles on his Crown Vic and swore he was going to take it to a million. However, the 4.6 liter V8 isn't going to be very frugal with gas.
8. No. Just no. See my comments for #4. Unless you know the car was well-cared for and unless it has a *full* service history with every scheduled service performed at the dealer... don't even bother looking at a Mercedes SL. Plus, if you think you're going to get 30mpg from a fifteen year-old convertible that weighs more than two tons, has the aerodynamics of a brick and a five liter V8... you are sadly mistaken. You'd be lucky to get 20mpg.
If your girlfriend wants a convertible, tell her to look for a used Mazda MX5. The best used convertible for anything less than ten times the price. Plus, because they're so common (there's a reason they are the bestselling convertible in history), the prices are low. Parts are cheap and plentiful and there is no shortage of people who know how to work on them. They're as reliable as anything on the road and the first two generations had cast-iron engines so it isn't uncommon to see them with 200,000 miles on them and still running strong. Even better, since your girlfriend probably wants an automatic transmission, you'll get a better deal on a used Miata... Sellers of used Miatas with automatic transmissions are terrified that nobody will want to buy them because they'll want a manual. Don't let on that you want an automatic (even if you do) and beat the guy down on a price a bit.
Also, look into a used Subaru. They're tough as nails, usually quite cheap and most of them have all-wheel drive which gives more confidence in slippery conditions. Plus, most of their engines don't use rubber timing-belts so they're more reliable in long term use. I'd rather buy a Subaru with 200,000 miles on it than a Honda or Toyota with 100,000 miles on it.
-I don't watch Fox 'News' for the same reason I don't eat out of the toilet