Peter
I think you would be suprised just how much you can get in a Z3. Mike Fishwick loads his for a 2 week camping holiday for two people, there was something about it in one of the club magazines a while back. Mike also posted this item on zroadster.net in the knowledgebase section
With regards to rust appearing, I find that a little strange unless the car has been repaired at some stage, most Z3's at the moment seem to be rust free, apart from a few spots round the boot lock, and most of these are sorted under the anti corrossion warranty.
Sounds a good price too for a 99, most models of that year fetch a little more than that, I assume it was not a BMW dealer forecourt.