The cooling system capacity is 11.6 quarts, so 1.5 gallons sounds about right for a drain and fill, although it may be a little less or a little more. If you are going to do just a drain and fill, I would buy the 50/50 premix.
If you plan on doing a distilled water flush (which I would do to get all of the old coolant out), then what you want to do is drain the system, close the drain, refill with distilled water, run it for a bit to mix the remaining coolant with the new water, drain, and repeat until the drained water comes out clear (probably 4 to 5 drain and fill cycles). Then add 5.8 qts (or round up to 1.5 gallons) of straight coolant (not premixed) and top off with distilled water.
i have had the coolant changed twice in the last 8 years,,,4 1/2 yrs the first time and a little over three years ago for the last coolant exchange.....it's dex cool, and it's very clean,,,i don't see a reason to do a complete drain, flush and fill again...unless someone else thinks that just a drain and fill is a bad idea...thanks