Save Deca for another cycle. Test alone is going to blow you up, because this is your first cycle. Save the Deca for future cycles when AAS wont have as much of an effect on your body, like it will on a first cycle. If you run Deca with Test, because both compounds seem to "kick-in" around the same time, you won't be able to know what compounds are causing what sides. What might feel like gyno could be a serious prolactin issue.
Water weight can be controlled by using an AI. Water weight can help with strength gains. This being your first cycle, you have no idea what kind of sides to expect from Test. This being said, you might not get ANY sides, gyno issues, water weight issues etc. But you might also get really bad sides or somewhere in between.
Dbol vs Deca is like apples and oranges, black and white.
Just run a basic cycle,
Week 1-12 Test E @ 500mgs/week
Week 1-5 Dbol @ 40mgs/ED
Your "goal" of 15-20lbs is a 110% realistic with this cycle. But, the ball is in your hands.. you have to provide the hard work and dedication with diet and training everyday. Make it a priority. I would guess, if you had a solid diet/training/recovery time, you could gain 25-30lbs, probably keep 80% + with a solid PCT. BUT I have no clue who you are and what level you at with bodybuilding.
*Liv52 for liver support. Clean diet, with surplus of calories. Diet is what determines your outcome. It is the heart of a cycle.
AI, Armidex @ 0.5mgs, when needed. For water weight and other Estrogen related sides.
Then if your looking to cut next cycle, or future cycles then run a solid cutting cycle and throw a nice stack in there, at this time you will be more educated and a lot more experienced.