USVI roads are tricky (fun!) and can be downright treacherous in places. As you probably know, cars get beaten on pretty badly there too. Both of your original choices were sensible although given the cost of fuel down there (both gas & diesel) you may want a smaller vehicle for daily / routine use.
I didn't see many stations that offered diesel as well - at least they sell "real" gasoline there (not the garbage here on the US mainland that they pass off as gasoline) but it's not cheap. I think it was north of $3 a liter a year ago but I'd have to check my receipts. I'm sure you can get diesel but your options are going to be more limited - maybe marinas have / can get it?
I love it down there. The east side of St. John is in my top 5 places I'm considering for retirement myself (I'm not saying what the others are since I don't want word of them getting out too much - they're largely off the radar of most people).
Nice choice and I love the pictures of your place too - brings back a lot of good memories and hopes for the future... Well done!