1/2) I would not do 7 subs - The budget on the linked install is VERY big - Whats yours? IMO I would go for a single high quality amp & sub.
3) With decent front stage and subs in the back IMO don't waste your money upgrading the rear speakers just disconnect them
4) Door speaker upgrades are a MUST if you are putting in amps/subs if you want a balanced system. Likes of JL 3-ways, Focals etc.
5) Head Unit - I would go for one of the newer Alpine models i.e. W502 or maybe the Kenwoods too. (Avoid Panasonic as they are closing up in the european car audio industry)
6) Handy to do as the stock one is not great.
7) Dynamat does not have great margins so even buying in bulk does not save you that much. Do a search for sound deadening and get reading. Lots of options.
8) Get over to www.talkaudio.co.uk - very knowledgable about whats on offer.
As you say you don't know much about installs - no of subs != better. You will often find the best systems out there are the simple but high quality ones. Might be worth getting to a meet to see what sort of sounds you can get.