
He won't like it, but I'd start with offering half and go up to maybe $4500. Argue that the equipment is not new anymore, and that if something breaks, you have no warranty ...or is he going to offer you one? A 90-day warranty could bump his price close to those of refurbished units.
Be sure to give yourself some good options, too. Look around on eBay for the same equipment. It will all be used and have different hours listed, but it should give you a good approximation of what a realistic price is.
I'm sure you won't find all three in one listing (Trailblazer 301G, 12VS Suitcase, and HF box), but you should at least find some Trailblazers and other Suitcase welders.
In the end, don't pay more than you'd be satisfied with, and don't find yourself 2 months from now thinking, "Drats! Why didn't I offer $200 more and get that package?"
My 2 cents!