First off.. you don't have to be a designer to use a mac. Get that out of your head.
Second, I've been using Mac's for 7 years now. Switched from PC's which I've used my whole life.
You will encounter a problem or 2 occasionaly, but it's much less than a PC. You also, at the moment, don't have to worry about virus or spyware. The computer always runs fast, providing you didn't cheap out on your ram.
Buy a 17" laptop and it's the only computer you'll need. I highly recommend it!
Photoshop (and not the less expensive LE or Element versions) is necessary for image editing. Get nothing else. I have not done one day's work without it in 10 years. Get it no matter what computer you have.
Not true. There are many opensource programs that give you the ability to edit images. Photoshop is not the only one, but I find it to be the best. It's also expensive, so if you're on a budget, you can download a free opensource program.
Viruses are possibly, but highly unlikely.
And ANY MAC will not do. This OS needs at least 1gb of ram to operate smoothly. Do not cheap out and buy a basic model. Trust me. Go for the 2gigs and you won't have to upgrade or buy a new model for a long time.
I believe GARMIN offers MAC support now, if it's not garmin, it's one of the other big name GPS guys.
Yes, you can also run a windows environment on a mac, in 2 modes. One is a complete bootup mode and the other gives you the ability to run windows while running mac osx. Honestly, once you learn how to find the software you need, you'll have no use for a PC. I have NEVER needed a PC in the past 5 years. Macs now have tons of software that works in place of a pc program. Plus, you won't have to go and buy a microsoft office program.