I have a '99 Boxster too. I have been happily upgrading for the past few months, without going over $3k. It depends on what you want to do.
I would definitely recommend doing the OBC hack if your car doesn't have it. Spoiler stickers are a fun addition, especially with the in-dash spoiler switch mod. There's also desnorking, hardtops, clear headlights, etc.
It's up to you what you want to do.
