I recommend getting a dildo that has a curve in it. It allows you to position the dildo's tip to rub against the prostate for maximum stimulation. If you find a dildo that has a head, or even better, a head and another bump along the shaft it gets even better. It's probably a bit bigger than you were looking for, but the Vixen Maverick shows this if you look at a picture.

Size-wise I'd say go for at least 1.5" diameter and 6" of insertable length. For many people about 7" insertable is the maximum they can comfortably take, as that puts the tip of the dildo near the end of the rectum. A wider dildo is a bit sturdier and also provides more stimulation.

Most dual-density dildos should work as long as they're at least 1.5" diameter. VixSkin and O2 both have a stiffer core that enables anal use. Stay away from the pure "supersofts" as they're harder to insert.

Speaking as a guy, a wife with a harness is a fantastic handsfree device. That may not be available to everyone, so...

The Tantus Suction Cup seems to work pretty well. It holds the dildo fairly firmly, and if the surface is smooth and nonporous it seems to stick for a while. The downside is that you need a vibrator pocket for it to work. If you're handy with a drill you might be able to modify a dildo to work with it. Other than that I'd say try boring an appropriately sized hole in a piece of Plexiglas or other flat plastic and use that to hold the dildo down. Perhaps a sandwich of that with a board on the bottom and straps to hold the whole thing down? Trying to use a harness on furniture usually doesn't seem to work well.