The "how would they do with highway speeds?" question has already been addressed, and I'd concur. Cruise at maybe 65mph with stock tires and gearing on level ground. Hills, re-gear, big tires, turbo, etc. all will affect this +/-.
My opinion of what to look for, since you asked
I'd look for LHD. I've driven RHD a lot, and it works great in Australia, but it got to be a nuisance for me in the USA. Others disagree, so there it is. In order of preference re: motors (if unmodified), I'd go: 1HZ, 1PZ, 2H, 3B. Any of them would be fine for almost all the driving you say you'll be doing. They'll all get the job done, and again, YMMV. There are other, arguably better motors, but I think these are all that might be available in a 75-series ute of appropriate age to import to the USA.
The other big thing to watch for is rust. They're likely to have it. You may get extremely fortunate and find a truck without, and I'm sure someone has, but you should be prepared to determine how much rust you're willing to live with and/or repair. Not all rust repairs are created equal.
Where to look? Limited options if you want LHD: Europe, maybe Central (and South) America, and the Middle East. Is that about it? Northern Europe usually = rust. Central America usually = rust. The Middle East usually = no rust, but hard to find older cars at all, much less ones that haven't been beat up. RHD = Australia and Japan. Not many utes in Japan, they mostly got SWB and MWB 70-series. LOTS of utes in Austalia, but of course, all RHD. If you get one from the middle of Australia (i.e. away from the coast), that usually = less rust, but lots of km's (it's a big country).
Hope this helps. Kind of makes it sound like a "needle in a haystack" search, which it kind of is if you want a nice truck, 25+ years old. I have found them, though, so it's possible.
