Dreamer44, your welcome. I'm glad you found it useful. Nearly all of the basic ideas are the same as fresh water aquarium husbandry. Good luck with the birthday present and if you're worried about having too much time a salt water tank will be more than happy to solve that problem.
The things you mention will indeed hold saltwater and fish, but don't underestimate the cost of the little necessities required for basic maintenance and the tools to solve common problems. They quickly add up in cost, and you'll probably need them sooner rather than later.
Use reverse osmosis(RO) or distilled water. It has little to no dissolved solids so you are starting off more pure than tap and there won't be any harmful chemicals in it.
A reverse osmosis filter to make your own water is the cheapest way in the long run, but it requires a much larger initial investment. You can buy distilled water at Wal-Mart by the gallon. It costs around $0.64 here. It could be more or less in your area.
If you want a price on a 12 or 24g tank with lights, hood, and powerhead this site offers them for sale and does not allow us to post competitors on the boards. Look over the variety they have to offer and the prices they charge for a general idea.
You don't have to have sand at all and in some tank situations you may not even want it. If you decide that you do, then you have to pick a grain size. Basically figure on something like $20-$30 for a 20lb bag. You can get it cheaper and pricier, but most of the readily available pre-bagged stuff costs around that and comes in a 20lb bag. Add sand to your liking. There are tons and tons of articles discussing the merits of this and that when it comes to sand.
Live rock is probably going to cost you around $7-$8/lb if you walk into a fish shop and buy it. A good starting place is 1lb per gallon. Aquascape it as you see fit and if you feel like you need more add it.
Don't forget a heater to keep your temperature from dipping and something to sit the tank on. Plan on your tank weighing something like 10lbs per gallon when it is filled with water and rock. Thats 120lbs for a 12 gallon, so be sure whatever you are sitting it on can handle it.
So for a really simplistic basic price:
12 gallon PC lit tank - $150
12lbs live rock - $100
bag of sand - $25
heater - $30
water - $6
Salt mix - $25
Hydrometer - $5
Light Timer - $5
[hr]
Total - ~$350
Expect to pay double that by the time you've got something worth looking at. There are also maintenance fees. You need to change 10-20% of your water at least monthly. You need to replace your bulbs every 6 months. Your fish have to eat. So be ready for the ongoing expenses.