Great idea to rent a car; even better to have someone drive for you so as to obtain the full pleasure from tasting at distilleries. Islay is a three-day tasting venture because the hours, generally beginning at 10 a.m. (Lagavulin is powerful that early in the day!) and last tasting at about 3 p.m. It is difficult to do all distilleries even in three days.
Because of the taxes in UK, the only whisky I bring back to the states is a version sold at a distillery that is a special edition and likely is not sold at retail outlets. I am not certain of the tax situation on bringing back duty-free whisky, but in recent years the bottles sold in duty-free shops at the airport seem to be only a few dollars below market in Atlanta. I have made some 20 trips to Scotland, but ceased bring whisky because it seems not worth the trouble unless the bottle is indeed special.