Since this is not the only project you will ever do I would pick a good all around rod that will be easy for you to use and perform well in all positions. I personally prefer 6011 for all around work. If I only had one rod in the shop it would be 6011. Good penetration and fast freeze. Not as pretty as 6013 but I feel you get a better weld. If pretty matters and since your welding flat then go with 6013.
Everyone has thier own preference and this is mine. Good luck and congrats on the welder
JWB