Install Gas Water Heater
 

1. I'm wanting to install a new gas water heater. I was planning (for simplicity) to add stainless steel flex water and gas lines from the existing lines to the new water heater. Can I do this?

2. I need to add a shut off on the cold water supply. I was thinking of a 1/4 turn ball valve that would connect the copper line to the ss flex line. Do I need a dielectric fitting in between that?

3. Other things I am missing or may need.

A permit? In many places you are not allowed to touch gas.

Just contacted my building inspector. He said he'd allow me to do the work myself. Is there anything that you see that would be cause for concern or not allowable by code?

You should have ran you plan past the building inspector while you had his attention.
If you actual connection to the water heater is SS then dielectric may not be necessary.
You could also ask if the flex gas line is acceptable.
And you could have asked him what your exhaust venting requirements are.

He okayed me on the flex lines both for water and gas. The only other thing he mentioned I would need is an expansion tank. My water heater is going to be 50 gal. Any ideas on how large of tank I'll need. I think the store only carries two sizes. One is something like 4.5 lbs and the other is closer to 9.

Try these link for the Watts Potable Water Thermal Expansion Tank Sizing Tool

A gas heater is also suppost to be sitting up on a stand to keep it away from the floor drafts and flamable fumes. It also should be sitting in a pan with a drain line leading to a drain line outside. When we install one where there's an outside wall with vinyl siding on it we install a siding block and run the pop off valve drain and the pan drain line to the outside of the house.
In CA it's code to also attach the heater to the wall with straps so it does not tip over in and earth quake.

The stand requirement depends on where it is being installed. But under many code now it is being dropped even when installed in a garage...

FVIR eliminated the need to raise the water heater off the floor to get above flammable vapors. Check your local code.

I do not think they should drop the requirement of a stand in a garage just because of the FVIR heaters. Yes FVIR is supposed to prevent the flame inside the heater from flashing outside the heater, and safely burn off the flammable vapors within the water heater. There is a few things that is flawed in allowing the FVIR heaters to be floor level in a garage.

1. Rheem and a a couple others have a glass vile that breaks when the heater starts to burn of the flammable vapors. Once it is broke the heater is useless
2. A.O. Smith and State have the fresh air intake on the very bottom of the water heater, most garage I been in are not the cleanest, so these heaters will act like a vacuum cleaner sucking all the dirt in though the FVIR screen and clogging it,again making the heater useless
3. Due to the above mentioned issues I have seen people open the sealed combustion chamber and or busting out the sight glass to allow air to enter so the heater will operate, but bypassing the FVIR so now the flame can flash outside the water heater

Thats just the tip of iceburg to say. When I replace water heaters in a garage I still keep the stand in place.

Hey I didn't say I agreed with it! LOL

But yes due to the additional level of protection afforded by the FVIR systems many codes are now dropping the stand requirement...