The obvious answer, lower mileage is better. If you can afford it, then get a car with lower miles, especially with your thought about selling the car as soon as your find a Caddy. A lower mileage car will be easier to sell.
Also the term "well maintained" is something to be scrutinized. Unless the owner has receipts from every dealership visit, every 3 thousand miles I wouldn't take the seller's word that it was well maintained. Also, well maintained has nothing to do with how the car was driven. You can religiously change the oil every 3k, but if the car is being redlined in every gear on a drag strip for most of the 3 thousand miles would you buy it for a daily driver?
IMO if you are gonna get something with a crap ton of miles, then why not get a project car? Spend maybe $1000-$2000 and put in some wrench time. I just saw a craigslist ad the other day for a 97 GT with a blown motor. The owner had already bought a crown vic mil for the replacement and was including it in the sale of the car. Apparently he just didn't have time to put into it. The guy only wanted $1000 for the car and all the parts. Granted this example will take quite a bit of wrenching to get the car to the mark where some of these 5-6 grand mustangs are, but you would have a fraction of the cost invested!
My last remarks on this thread and then I'll leave it be. If you are set on the CTSV, why are you buying a Mustang? Unless you find a fixer and put in the blood, sweat and tears into it to turn a profit, I don't see the benefit in buying something for the short term and having to sell it for a loss in a few months. I absolutely love my Mustang and I wouldn't trade it for anything, unless it was another Mustang (Fox Saleen or 69 Fastback or Termi). I guess my blood runs Ford Blue and I don't mean to discourage you from buying a Pony, but it sounds like you are set on the CTSV as your dream car. Don't buy a Mustang to use and abuse, buy it because you love it, wanna tuck it in at night, treat it right and love it until the end, not to sell it again.