^^^ $6k is a very decent deal, assuming your trade it isn't worth more than ~$2k. I agree about your 60mph, in fact I'd say that anything less than 70 is heading for trouble.....they really fly on the freeways
I have a '12 missing 2 bars(like the one your looking at) and with a 100% charge(actually doesn't go past 92%) I can get 60 miles at 65-70 mph with no problem in the summer, with much heat use in the winter(I live in the fridged north) that could drop to < ~40 miles. I'd also strongly suggest Thor's heater mod switch, without it you really have no control over when the energy hog heater comes on, if the temp is low and you want air to possibly defog/defrost the windows.
The '12 only has a 16a 240v L2 charger so expect only ~15%/hr of charging, of course a DC QC can get you to 80% in ~30 min. but they aren't as readily available as L2 or even L1 outlet chargers.
Once you get it you can play around with trying to get by with a 80% charge for your 40 mile commute, it would probably be close and would depend quite a bit on your driving habits and of course heater use.