gas water heater DIY install - also water hammer issues

So I am looking into doing a gas water heater install... in the attic. I've received a couple of quotes that are within 50-100 dollars of each other. Highest quoted price right now is $1200 plus $350 for installation of a thermal expansion tank.

First question - I know the steps of how to DIY install this. Getting it out of the attic is a pain, but I think I can manage with a friends help and chain hoist. I understand the risks with gas, and not venting the flue correctly, etc. My question is, is it really easy to screw this up? I would LOVE to save the $500 or so to do this myself, but don't want to kill myself in the process. Thoughts on the DIY attic install? This would be the biggest plumbing project I've done so far, and I've never really dealt with natural gas or CO.

Second question, I have water hammer. I was planning to fix via thermal expansion tank. I plan on sourcing the thermal expansion tank from home depot or lowes and have the plumber install. Is this what I should buy? If I end up doing WH myself, then I would DIY this as well. Should this fix the water hammer?

http://www.homedepot.com/p/Eastman-2-gal-Thermal-Expansion-Tank-60022/205364872

Thanks for your thoughts.