Any of the Britax seats with ClickTight are really easy to install with the seatbelt. That may be your best bet as you don't need to worry if there are lower anchors in the seating position, don't have to worry about them not getting the seat tight enough as ClickTight does all the work, don't have to worry about whether the seatbelt locks or not or if the driver knows how to lock it or remembers to lock it, and while using the top tether is optimal, I think it can be used without in the USA until 65 pounds. I'll go double check that though.
I seem to recall one of the techs here said a Click Tight install was really quick. Like under a minute quick. They will need to take time initially to learn the install though.
The Frontier 90 would be the cheapest model with Click Tight and converts to a booster when she outgrows the harness.
ETA: from the manual it appears that the seat can be installed without top tether until 65# regardless if you're using seatbelt or lower anchors