First thing is, don't use sand. It causes silicosis in your lungs. The two products I regularly use are Aluminum Oxide and Glass beads. The alum oxide is for general blasting and the glass for soft metals like aluminum. The body shop I go to used soda on the '73 Mach 1 body that I'm just finishing up. It worked well and seems to be a lot safer/easier to use. I'm going to switch to it for my bigger parts that I clean in the back yard. I side benefit seems to be that the soda leaves a finish that is slightly resistant to rust, so you don't have to shoot primer on the metal as soon as you've blasted it, as long as you're inside and protected from rain, dew, sprinklers, etc. As for what to look for, buy something that has a good supply of parts, that you don't have to wait for the boat from China to arrive to get. Both my blasters are from Harbor Freight and I'm going to upgrade this summer. They work well, but I'm tired of waiting 6-8 weeks for nozzles when I wear them out.
Steve
PS. Make sure you get a hood with air supply. Nothing you breathe while blasting is good for you. The other alternative of course is to just pay for someone else to do the work. A lot of soda blasters will come to your location, because they don't have to worry about clean-up.