upgrade help

Hi, okay, now my questions....I am new here and looking for some advice. I have a 40 gallon with a few fish, and quite a few corals,the tank is pretty full, been set up around 5 years, maybe it has been longer..... I am looking at a 90 gallon used set-up, he has had it 5 years and bought it used. That comes with 3 fish,2 tangs and a clown, and just a few corals, good metal halide lighting, skimmer, stand.....basic. I would like to turn my 40 into a refugium for it. From what I have read, I should buy new dry sand. My sand is old and his is old. I understood mostly on taking the 90 down and the transport, fish in a bucket, live rock in a bucket, some of his water, 1&1/2 hr transport.

Since I am trying to put two tanks together, what would be the best thing to do? Use my saltwater from my tank? and my own salt brand mix? or should I use a combo of his saltwater and my tank saltwater? or mostly all new water? Should I put just his stuff in the tank first? add mine later? or try to do it all at once? Or would I be making a mistake trying to put two together? I can't afford to just go buy everything new for an upgrade, and the deal seems pretty good, and he is pretty desperate. He has moved and needs to get the tank out of the house he sold. My space is extremely limited also. I love my tank.....really love my tank, I don't want to lose anything, but have been dreaming about having a bigger tank, and it just seems to be in reach. But I don't want it to turn into a nightmare either.