Hello BCReefer...
Number of things to consider :
1) When buying a tank, test tank for leaks with fresh water. Test in area where a leak doesn't wreck your house.
2)Top off for new tank should be done with saltwater such that the total salinity of your tank is about 1.021 to 1.025.
As for tank sizes, try to find a tank that isn't very tall and very shallow. Something along the lines of >16" deep and 20" high is acceptable. Tall tanks are difficult to light up and difficult to perform maintainance on the bottom of the tank (i.e. I am short...). Shallow tanks are difficult to arrange lr.
Any who...these are guidelines and suggestions. Any tank will work...just some tanks are easier to deal with. Drilling two holes in your tank is also suggest so that a sump can be added immediately or in the future. See link for sump explaination :
http://saltaquarium.about.com/librar...y/aa061900.htm
P.S. Take your time and do it right the first time. If you don't have the money now, then wait till you do and get the quality equipment you wanted. I've wasted money on buying cheap inadaquate equipment and later regretting it.
- Victor.