I think jim32190 answered most of your questions. Just a couple of things to add:
1. Dream cars are NOT a part of the $25 upgrade. Sometimes there are some decent vehicles worth upgrading to for $25/day, but I don't think anyone has ever gotten a Porsche Anything for the $25 upgrade fee (speak up if you have).
2. With UC it's probably not worth using an upgrade coupon. Better to use a $-off or %-off coupon instead as you will have much better value with that, especially as PC.