I would definitly scratch the 40g Tall off the list of optional tanks to use, you're better off with one that has a larger surface area which will allow for better gas/oxygen echange to take place in the water. I would personally go with the 55g if this is going to be your first SW tank as the great the water volume the more room for error in water chemistry.
As for cost of setting up... that is a tough question to answer unless, like as stated above you can give us an idea of what kind of tank set-up you are shooting for. It could be anywere from $500.00 - a few thousand dollars, my new 20g w/15 sump & refug. is probably worth somewhere in around the $2000.00 dollar range, and that is not including the cost of test kits, Salt mix and stuff like that. One word of advise when you go buy your Salt mix spend the money on one of those big buckets of mix that does like 200g, it is alot cheaper in the long run and your going to use it up quickly enough anyway so it saves you extra trips to the store