AFRay007,
When I bought my Boxster last year I had the same problem ( although I'm sure that winters in North Dakota are much worse than here in Chicago ).
I thought about buying winter tires & wheels and a hard top, but I decided to just buy a winter car instead.
Figure about $1000 for wheels and winter tires and $3000 for a hard top - you could probably find a nice 4x4 for $4000.
Not sure about North Dakota, but there are some awesome roads in South Dakota !
That dealer does look scary, though. $35k for a 1999 Boxster with 38k miles - $50k for a 2000 S with 23k miles
Good luck...
Nick