I would just run test, but don't expect to be super impressed on 500mgs of test. Don't get me wrong - 500mgs WILL build muscle well, but if you really want to get a feel for what kinds of gains are possible on test, you need to run it higher. My point being, that if you were going to run 500mgs for an extended period of time, then the increased anabolism of 500mgs would get you to your goal, but if you want to add a mostly solid 20lbs in your time frame, you'll need more anabolism than 500mgs of test by itself will give. Testosterone has a dose response relationship - the more of it you take, the bigger and stronger you can get.
"IF" you have used testosterone before, then I would start at 250mg, and increase by 250mgs per week until you hit 1000mgs and run that for 8-10 weeks.
That will put some muscle on you and be both the most cost effective and healthiest option.
I don't recommend deca because the potential sides are harder to deal with than basic estrogen sides.
Make sure you have Arimidex or Aromasin on hand, as well as Nolvadex. I cannot recommend a specific amount of ai to use because ai is the most individual thing for each person - cookie cutter recommendations are shit as far as how much aromatase inhibitor a person should use for a given amount of aromatizing steroid.