I think it's a very British thing to place so much credance on mileage. Most other countries, especially those with kilometres as their standard unit of length, seem to look more at condition and history. For example, my sister today bought a 152,000km Volvo XC90 in Berlin., after talking to my bother-in-law, he was not at all put off by the mileage and thought it strange that I asked. Coincidentally, my XC90 just passed 100,000 miles and is totally fine. She's 11 years old and perfectly maintained and although it prompted a discussion with my wife about getting a younger one, I can't see an decent argument for doing that.
Previous to my GS, I took a gamble on a 90,000 mile c4s which I took to 105,000 without incident. Early 90s 911 964 are now commanding 20k even with 150k on the clock! For me, the real decision is whether you want a low mileage car to cherish as a Sunday driver or whether you just want a good, often substantially less expensive version to use and attain the same enjoyment from, if not more.
I've done the depreciation thing and I dread to think how much my motoring life has cost so far. Unless the money element becomes less significant for me, I think I'll stick to higher miles and keep the costs down.
To summarise, a 66k GS is not high and at 18k is a bargain to my mind. It may well be harder to move on but it should suffer low depreciation only and give many smiles per buck. Seriously, what else come close at the price?
....or is that a good thing??!!