Welcome to the addiction
I am on a similar path. 3 years ago I set my sights on Dakar, at the time I planned on doing it in 5 years (2015) but a recent racing injury has set me back at least a year... probably more to be honest since I don't think I was really on track to do it in 5 years. Sounds like you are starting with more riding experience than I did, and most certainly you are starting
from a better level of fitness.
I started a thread back when I started my goal, not it orignally said 10 years as the goal then I later changed it to 5. I suspect the reality will be somewhere in between.
Also I am documenting my journey in my blog: http://www.dakardream.net/
You seem to have a good understanding of the cost, that is important as a lot of people get scared off right away when they realize what it really costs to do this event.
Events I would suggest:
Mexican 1000, (I think they have a quad class, not sure). This is a 4 day rally through Baja. Lost of vintage vehicles but also open to modern vehicles. Fairly lightweight compared to other rallies but a great way to get started.
Australian Safari Rally. I believe they might be doing free shipping from N. America this year as part of the entry fee. Several ADVers have done this in the past so should find plenty of info if you search here.
Best of luck!