Just to answer the question I posed above for future use, I eventually put together my own middle-of-the-road definition. There are no absolutes, so I'll generalize.
The stock wheels didn't vary much in width, going from 7 in the front to at most 9 in the rear, and generally 1.5 inches wider in the back than the front, so I tried to honor that difference. A width of 7 in the front would mean 8 or 8.5 in the rear. I didn't bother considering the same width front and back as Porsche did once with snow tires, as I don't intend to drive in the snow. That would make front-to-back tire rotation a possibility, though.
The offsets were most often between 52 and 65, (again only higher at 68 for winter wheels), and the majority of wheels you'll find in the aftermarket are offset 55 so that's not too much of a problem to deal with. Lots of the Boxster and Cayman wheels are offset 48, which may require a 7mm spacer on the front, and which arguably requires longer (52mm) wheel studs and a (recommended anyway) swap to steel lug nuts. I believe H&R Trak+ DRS is the series of spacer that includes the studs. Their DR and other series use bolts or come without the necessary hardware.
Compared to 18's and above, 17's are much harder to find unless you want a set of Turbo Twists (no thank you) or something off a Cayenne that looks vaguely like the original 924 Arachnids (ugh). Finding a good set, at a good price, has taken months, but is worth it if you want to modernize the look of the car just a little and you don't want to overtax your handling bits. The C.U.P. 2's are quite common, but the price ranges tremendously and many of them are beat, so buyer beware.
Definitely read the comments and ask a lot of questions about wheels before purchasing online. At least 75% of wheel advertisements do not include the offsets and you will have to ask (and most likely have to describe how to find the offset to the seller). At least 50% of the wheel advertisements incorrectly list the part number or the actual diameter of the wheel, or the heading will list one size and the description will list another. And this is from sellers who specialize in wheels!
