You might be able to get someone to come out and do just the footing... I would ask even places like home depot, menards and the companines you already talked to. A lot of places are willing to do just part because that means they still get money. Of course you will get hit big time with labor costs so in that case it may make sense to just have them do it all.
A good way to learn how to do it is check out some books from the library about building decks or go to a book store and look in the home improvment section for building decks.
We redid your roof, neither bubby or I have ever done it, both of your dads had so we did get help from them. We also did have friends come and help who have never done it before and really anyone that could follow instructions could do it (just took time, like 2 weeks to do it yourselves).
If you do it yourself be prepared to spend many weekends doing it (dpending on how big it will be), it will take longer then having a perfessional coming out to get it done in a few days. It does save money doing it yourself, so sit down and do your homework on how to do it then decided if it is something you can do.