might be code in your area to get a permit when replacing a water heater, might want to look into that. If you have home insurance they should cover a good deal of the price of installment by professionals. It varies widely from one insurance company to another, so look into that.
As a plumber I can say the average water heater replacement when home insurance is involved comes out to around 300-500.. not incredibly horrible considering labor, materials, and other costs can add up to $1000 or more.
If you do give it a go yourself it really isn't a difficult job with some general plumbing knowledge. I'd say a 3 out of 10 difficulty without counting picking the old one up and getting it out to the trash and getting the new one up and in place as a difficulty..
You pretty much got it..
Depending where you are might need earthquake straps (I suggest them no matter what, just keeps the thing sturdy and in place.
A nice stand/foundation for it to sit on, if it's leaked all over the existing one and it's made out of wood you might want to look into something new and metal.
It might be code where you are to have a drip leg where the gas line meets the water heater
A pan under the water heater is a good idea.
Watch out for the vent at the top. They are notorious for tearing to shreds while trying to install a new water heater.. If you're careful you'll be able to use the existing venting, change the hood.
like I already said look into the need of a permit, could cause problems in the event you sell your home... could also cause problems with the warranty of the water heater itself.
oh and drain the old one real well before trying to move it.. Hose connection at the bottom, screw a hose on there snake it to the driveway/front lawn open it up let it sit until it stops draining. Shake the crap out of the water heater to move any sediment out of the way and let remaining water drain, if still dry then move. (I know that seems obvious but I've seen plumbers in they're first month on the job who've installed water heaters forget this step and pay for it with their backs..)
I think that's about all you need to know, but if you have home insurance once again I say you should talk to them first, see what they say.. We just had ours go out, I was going to do it myself.. as a plumber the hardest part would be getting the thing here on my day off.. But our home insurance took care of it for about $280.00
good luck!