First off, welcome to Fishlore!! I'm sure you'll find a wealth of info here.
Some of the things to look for at that tank at Wallmart. Make sure the heater is
SW compatible. What kind of filter does it have?
The biggest expense in a SW tank is the
LR used for filtration. There are ways to cut down on that expense though. Get mostly base rock and a bit of
live rock. The base rock will convert overt over to live rock though it will take longer. The biological filter part of it won't take that long though...just not much coloration for a number of months. You can also add half as much LR (live rock) that you'll need in the end and then add more as you can afford it.
Another good thing about a larger tank is that it's easier to maintain water parameters.
Don't get the water from Walmart or Petco. Find a good LFS who sells water in bulk. Here in metro Atlanta, most of the stores sell it for $1 per gallon of salt and $0.50 for freshwater (to replace water due to evaporation). As for getting it home, you'll nice large containers. The store I deal with charges $10 per 5 gallon jug. Needless to say, I spent a lot initially but after that it wasn't so bad. Now if you find other containers of decent size then that may work.
I do get some of my equipment from Petco or Petsmart since it's cheaper than my LFS but I only buy my livestock from my LFS. Some of their corporate stores are better than other when it comes to Petco or Petsmart but both don't exactly have great reputations in general. The LFS that I deal with has high quality livestock. But then again you have to check out each of those stores and see what's what.
Hope this helps.