For what you describe:
fits in your FJ, large enough for 2 adults, dog and cooler I think something in the 10 foot range is what i would look at. Mine is 9'6" and four adults plus cooler fit comforably. For the outboard: max HP depends on the boat but you can figure 10 HP max for a boat this size. Those with 25 HP are most likely RIBs (fiberglass bottom) and much heavier and don't fold up and much more expensive.
Inflatables in this size will be hard floored (wood or metal inserts) or a high pressure floor. With a dog, don;t thing high pressure is a good choice.
Boat material is PVC or hypalon. PVC is less expensive but must be stored out of the sun. Hypalon is more heavy duty and UV rays don't kill it. lots of info here"
http://www.westmarine.com/webapp/wcs/stores/servlet/westadvisor/10001/-1/10001/Inflatable-Boats.htm
Used prices? no idea as very few used ones show up. I bought both my boat and Nissan outboard from internet sites and saved quite a bit off local prices. With the economy now, locals may be more willing to deal however.
As to outboards, IIRC nearly all the outboards < 10 HP are made by Tahotsu, even if sold as Nissan or Mercury. Hondas are very good but pricey. The only 2 cycles available (as of a year ago anyway) were old stock as not legal to manufacture or import them anymore and the only ones available were larger HP. If you just want to dink along a 4 HP or < will move the boat. If you want the boat to plane, go with the biggest HP the boat will handle (or close to it anyway). Again, the heavier the boat, the more HP you need to plane it.
I have a 9'6" high pressure with a 6 HP Nissan 4 cycle and am very happy with it.
As someone else mentioned, I inflate it (using a compressor) and tow it to the boat using a small trailer. There is just no room to store it on the boat and inflating by hand is a real chore.