Fireplaces, Heating Stoves, Flues and Chimneys - Installing gas fireplace insert

I would like to install a gas fireplace insert into a currently wood burning chimney in a home built in the 40's. I am looking at a direct vented model that vents out through the flue, using two 3" co-linear liners and produces from 20,000 to 32,000 BTUs / hour. I will also need to install gas and electric. The fireplace has an ash door with the removal door in the basement. Could both the gas and the electric be run through that opening? I am interested in doing as much of the work as possible myself. I have some experience having plumbed and wired a remodeled kitchen, two baths and a garage with attached porch that I built. Is it realistic to consider doing the installation myself if I hire someone to check everything prior to actually tapping into the gas line to make sure it is done correctly, and to make the final hookups?

Also, one unit I am considering is rated: Steady State Efficiency: 80 – 88%, while the other one has a 75 - 87% rating. How much of a difference does this rating make and what does it mean?

Any input would be appreciated. Thanks much!

I would check out the code there for gas lines and electric lines first to see if you can do it the way you want.I dont get how you can have the different ## for the units 80- 88% or 75-87% a heater or furnace will have like a AFUE of 80% or we have some that are up to a AFUE of 93.0%

AFUE= Annual Fuel Utilization Efficiency
its the efficiency of a gas furnace converting fuel to energy ;) ED

After checking codes and reading the manual, I decided to leave everything but running the electrical to a pro. The manual was replete with warnings of disastrous consequences for errors and since I knew next to nothing about some of the warning subjects (reverse pressure situations, for example -- I know what it is, but have no idea what to do to prevent it), I thought that it was best left to someone who understood the warnings without having to look up key terms.

Thanks for the input.