I usually only read these forums and not post much, BUT, you're in the EXACT same position as I was about 3 months ago. I was looking for a "sport" sedan. Nothing from the states got my blood pumping. I read too many horror stories about German offerings, but I narrowed it down to either a BMW 540i, 740i (didn't want the iL), or an Audi A6 V8. I couldn't find an A6 V8 in my price range (also 15K), so that was a quick decision. I found a really nice 98 BMW 540i for 18K w/only 52k miles and liked it when I drove it (test drove it twice), but some of the LEDs on the dash were already fading, and the dealer wasn't willing to deal on it (in all fairness, he was asking quite below "avg book"), so I didn't get that one. I test drove a 2000 GS400 w/105k miles and liked it too, but the dealer was asking way too much I thought (19k I think). Finally, I found a private owner (i.e. no tax to pay!) that had a 98 GS400 w/90k miles on it w/new tires and windshield. This thing was seriously one of the nicest used cars I'd ever seen. He was asking 15,500 and took 15k for it. This is my first, but definitely not my last Lexus. I love the 400 (could use a BIT more power though). I've even been trying to convince my wife to sell her '03 Subaru Outback for a used GS4; she's so far resisted my idea due to being scared of a higher mileage car. We'll see how it pans out in the long run, but for 15k, I couldn't go wrong w/this one I didn't think. Even if I end up wanting to sell it in a year or two, I won't lose a bunch of money...some but not a ton I don't think. Good luck. Go for the GS400, you won't regret it.
P.S. A friend of mine has a nice 98 540i that he swears by, and it IS nice (I like the BMW body and suspension more than the GS), but in the end, I decided I don't want to pay for a BMW and drive a Pontiac rental every weekend while it's in the shop. I work w/an ex-BMW mechanic and even he told me to stay away from BMW. My friend w/the 540i claims he's never had any problems w/any of the 3 he's owned, but he's the only one I've ever heard say that. Hope this helps you a little. Good luck.