As already pointed out, you should check on the number of and locations of the D-Star repeaters before jumping into that arena, I do know there are a few in the UK as I've talked to a couple hams from there via the D-Star gateway...(I can also tell you with a lot of certainty that it's even bigger in Australia)
Using an HT as a mobile is a pain in the ass, if you do a lot of inner city driving the tall buildings will kill that 5 watt signal, if you drive out in the country the trees will do the same. The key to maximum distance in any VHF / UHF radio is antenna height, and since you can not achieve very much antenna height in a mobile application, you have to make up for it with raw power. A mobile radio with the ability of producing at least 50 watts for when you are in a difficult location, and 25 watts when you are within range of the repeaters is the best approach for a mobile radio.
Another thing to think about when using an HT (or any radio) for a mobile application is ease of operational use. I have never owned or operated a Yeasu VX-8R, but knowing all the functions that radio is capable of performing, I would tend to think it could be a little on the complicated side for mobile operation, and any distraction while driving can be a serious issue....in some states here in the U.S. if you get into an accident while being distracted with an outboard device...(cell phone, CD player, GPS unit, 2-way radio)..... it could lead to extended legal issues.
Based on the fact that I prefer Icom products, and I'm into D-Star, I would tend to go in that direction first. Although it's not on your list, I would recommend the ID-880H D-Star mobile radio, however if you are set on using an HT I wouldn't get the 91AD since it tends to get very hot while transmitting on high power, and you can not get a GPS speaker-mic for that unit. The IC-92AD is a better radio all around, it's built like a tank, water proof at 1 meter for 30 minutes, doesn't get hot like the 91AD, and has the option of a GPS speaker-mic that can be used for APRS applications, and of course has the D-Star digital voice and messaging.
If D-Star is not in the cards for you then I wouldn't count out one of the APRS ready radios from Kenwood either, they may not do as much as the Yeasu VX-8R, but they may be simpler to operate, especially as a mobile unit while driving.....