You ask some important questions and I am sure you will get some helpful answers from the more experienced uses with these platforms and the type of site you plan. I would comment on only one issue, the choice of a host.
I am not experienced with BigRock so this is no comment on them, but the cost of hosting is pretty competitive and the difference is not significant (unless you choose one of the specialized hosts that charge large amounts). Considering your investment of time and other inputs, the cost of hosting is small. Choose a host that you can determine has adequate staff and facilities to offer redundancy (backup) and support services.I'm thinking of BigRock, because it is cheap.
I have only used Dreamhost and Hostgator myself. Actually I have used another I won't mention but you do not want them! These two are reliable as are some others. BigRock may be OK, but make sure they are reliable first, then consider cost.