Christchurch to Nelson is around a 4 hour drive non stop. (probably a bit more - it's a few years since I've driven it)
I wouldn't suggest staying anywhere other than maybe Hamner on the route through Lewis Pass. But there are other options, such as driving through Blenheim & Kaikoura or down the West Coast - you are probably a bit tight for time to drive right down to Haast, but you could certainly drive back across Arthur's Pass.
It really depends on what you want to do and how much time you want in Nelson & Chch.
As for antiques. Most of the places along the way are more craft type places than having antiques. You might find some interesting things on the West Coast and Christchurch certainly has a good selection of antiques shops.
Remember however with antiques that NZ is quite young - only really colonised by the British generally 1880 ish. Maori artifacts which are older are in general not available for sale. There are items that have been brought out by British or Europeans that are older however.