It depends on how you want to use it. The AIR-S is the handheld unit, best for wheels, grilles, badges, mirrors and door shuts where water sits and drips later. The AIR GT and AIR-RS are the larger dryers built to blow the whole car dry, which is where the time saving really shows. If you already dry with towels and only want to stop water weeping out of trim and wheel nuts afterwards, the handheld is enough.

It is worth it for two things: touchless drying and getting water out of the places a towel cannot reach. Every towel pass is contact with your paint, so blowing the bulk of the water off first cuts the number of passes and with it the risk of marring on soft or dark paint. It also clears the trapped water in mirrors, grilles, badges and wheel bolts that otherwise runs down a clean panel ten minutes later and dries as spots.

You can move air with either, but neither filters the air properly, which is the problem. A garden blower pulls in dust and grit and fires it at wet paint, and a reversed vacuum does much the same. Purpose-built car dryers filter the intake and deliver warm, dry air at a pressure suited to bodywork. If you are drying a car you have just spent an hour washing, blowing debris back onto it is a poor trade for the money saved.

No, because nothing touches the paint. That is the entire point of touchless drying and why detailers use blowers on show cars and freshly coated paint. The two sensible precautions are to keep the nozzle clear of panels so you are not resting it against the car, and to make sure the intake filter is clean so you are not blowing collected dust back out. Used sensibly, a dryer is the lowest-risk way to get a car dry.