Welcome to FlyerTalk,
bjoerndd!
I assume you're going to Orlando or Miami, which are centrally owned corporate stores. (Dollar in Jacksonville is a franchised location--privately owned and licensed--and may have different cars. I'm not sure about Tampa, Fort Lauderdale, or any of the other locations.)
If that's the case, a Charger or Mustang is fairly likely, since these locations stock these cars. The Challenger is not: Dollar does not (to my knowledge) stock the Challenger at any of its locations.
Ask nicely, and since you'll be buying the coverage (in the U.S., the rental rate does not include comprehensive or collision [damage to the vehicle] or third party liability [property damage or bodily injury/medical to anyone you hit, so you must obtain these products separately), you can sort of "bribe" the agent (since they operate on a commission system) to give you what you want. Check the lot before going in (if possible)--that way, you can sort of have your eye on a particular vehicle and let the agent know what you'd like.
A Mustang convertible is also an option--I had one a few years ago down that way. They may want more money for one of those.
The ones you'll get, though, are not the massive motors that you see in the motorcar magazines--Dodge's 5.7L, 368-hp (274 kW) V8 Hemi or the SRT-8's 6.1L, 425-hp (317 kW) V8 Hemi or Ford's 5.4L, 540-hp (403 kW) Modular V8. They're usually the base trim--a relatively mediocre 3.5L, 250-hp (186 kW) V6 in the Charger and a 4.0L, 210-hp (157 kW) Cologne V6 in the Mustang. Those may still be big engines by European standards, but since the American cars are a good bit heavier than most anything a European sports sedan maker puts out, they're not as exciting to drive as, say, a BMW 335i.
While expensive, Dollar's
Style Series at Orlando may be something you're interested in. These actually
are the nicely-equipped, powerful versions of the cars listed.
Hope this helps, and please do ask any more questions about your trip! There's a whole sub-forum here devoted to Florida as well as sub-forums for the airlines you'll be flying and for the hotels you'll be staying at. People here are very helpful!
