The IMS you have is probably (if original engine) the best of the bunch.
You are buying a 17 year old car and there are parts that age as well as those that wear. Particularly things that are plastic.
Any way of telling how frequently the oil was changed? Any other maintenance history?
The hardtop is worth maybe $800-1400 depending on the season.
Age and condition of tires? That is a $800-1,200 item.
Condition of soft-top?
Do you have access to an experienced water cooled Porsche mechanic?
The person selling is valuing the low miles at around $6k as there are lots of higher mileage 97-99s at the 7 to 8k$ price point.
More than you ever want to know about buying a Boxster
here.