I'd say buy it if it checks out ok. Sure, the gas mileage will be worse than your diesel and probably your Taurus (I used to get 19-20 mpg, but after installing Yokohama AVS db performance tires, it has dropped to 18 due to my inability to keep my foot off the gas while cornering.
), but it's a big, solid, safe, quiet car.
Having also owned a Taurus, I'll mention the 300TE is MUCH easier to work on and doesn't seem to break as often. IMHO, a Taurus with 90,000 miles on it is well on its way to being worn out whereas the 300TE (and any other Benz) is just getting started.
I have found the wagon (and my other 300E) to be extremely reliable, and when problems do occur, the help at this site is superb!! (I've never come across www.taurusshop.com
I think a 300TE would be perfect for a daily 100 mile commute as long as you're ok with the fuel mileage. It's comfortable, quiet, and has adequate passing power. BTW, the engine is a inline 6.
Make sure the air conditioning, transmission and sunroof work perfectly as these are expensive to fix. About $2000 to $2500 each if they don't work. Complete service records are a huge plus to verify the car has been well maintained.
Having recently purchased a '91 300TE with a little over a 100k on the odometer, it seems $14,990 is about right if the car really is perfect. I'd start subtracting if I found scratches in the paint or more than just one or two door dings. If it needs tires, console or dash wood, interior trim pieces, a new radio etc, keep subtracting. If any of the big 3 (previous paragraph) don't work, consider passing on the car or dropping the price a bunch.
Hope this helps.