Very true 
but as said by my friend we do get help,from experts, but only once in a while..
we have planned to buy 2 arduino AT mega version of the boards, and can arrange for more..
but as you said the big project is a assembly of small projects, so we will be planning to do it as you said(doing the small ones first and then integrating them)..
But will it be a viable project to do if we buy pre-assembled smoke detectors etc.. or should we build them too.(we have enough knowledge to build them), but will it be viable taking the space constraints??
we have around 2-3 months to complete the project.. 