My 02 cents' worth:
The traction control on my car is quite helpful, especially in slick weather (fresh rain over oil-laden asphault when it's been 90 degrees out that day)
The upgraded stereo system is a must in my mind. If it has no door speakers, keep looking the upgrade path without them is very, VERY expensive and you'll wish you had found one with more speakers.
Get an S. I have a 2.5 non-S and wish I had held out for the 2000 S I dreamed about.
Tip or Non-tip for me meant the difference between me enjoying the car and my wife driving it FAR more than I and ignoring my demands to keep it out of grocery store parking lots. She has no passion for cars and could not care less if they're abused in parking lots. So no Tiptronic for me
More than anything that's been shared, what you must look for in a used Boxster is a car that has been cared for throughout its life. Mine was neglected and while I snagged it for 15k, it's taken 6k to get it in shape, including a transmission swap. So, my bottom line is this:
Buy a Boxster S with some factory warranty on it and find a good independent Porsche repair facility to fine-tooth comb over the car and pick at every flaw it has before you buy it. Then take it back to the sales person and tell them to fix everything on your list and you'll buy it. Basically, force Porsche to repair and replace anything wrong with the car before you buy it, and then take it back as often as you possibly can to drain the life out of that warranty. Oh yeah, not to scare you off, but if you're offered a warranty the dealership sells you for the years and miles after the factory warranty expires, seriously consider it. You'll probaby be glad you did.