Lori,
Block the home first to support the addition weight.
In doing what you mention you will have a problem building out because the side walls are setting flush with the edge of the floor deck.
I once took took a used 14 x 70 "trailer" home and completely remodeled. It started with a window air conditioner that had allowed water to leak around it and rotted the wood and floor. I removed the unit and opened the opening up for a new vynal slider.
That window didnt match the others so I decided to make them all match. I removed all the paneling in the entire home, made all the opening bigger for new windows.
Knowing I would have to replace the paneling with sheetrock, I knew the flooring would have to go and I couldnt have a old kitchen either.I purchased new oak cupboards, tile countertops, appliances and garbabe disposal. I stood new 2x4 studs against the old studs, ran new wiring and replaced all the wall insulation. The old bathrooms had to go too at this point. Cant have a new remodel and an outdated bath.
I worked on the home on weekends after being gone from home all week. "About killed us". 2 years later, I had a new looking home that looked great from the inside. So, anything can be done. If your like me, or many---one thing leads to another and another. If your the kind of person that can start one project and see it through to the end, your in great shape. Others start, start another, and another and another....You get the idea!
Good Luck!
Please email me pictures of your remodel if you strart. I will put them online for others to view.
-Tracy
www.mbqc.com