Lots of great advice here for you. There are true sports cars and then there are sporty cars. The 986 is a true sports car, you won't regret driving one. With an 18K budget, you can get a great S model for less than $15k. Take your savings and get the IMS taken care of, if it hasn't already, and then drive it like you stole it
There is no comparison to the 350z IMHO. I drove a few 350's in my search and it is a nice car, but it is not a Boxster.