What are your parameters for "older" vs. "newer"? Are you thinking you'll be actually travelling a lot of the time on this trip, or will you more likely spend a week or two in one place and then move someplace else for a spell? (If it's traveling frequently, a motorhome has some distinct advantages--you have a lot more space and ready access to conveniences while on the road. If you're parked at a camp site most of the time, those advantages are much less important.)
I think the most practical might be to by a (somewhat) older, good quality motorhome or trailer that does not need gutting and refurbishment and using that. At least to me, rennovation implies that the unit is in bad shape to begin with, which for RVs very often implies a good bit of water damage and a whole lot of work to bring it into decent shape. (If you merely mean redecorating, that's a little different matter altogether...but still start with a unit in good shape.)
If you want to do a lot of customization or have neat ideas for a layout or something like that, it might be better to make a schoolie (school bus conversion) or similar. That's a lot of work, but you're at least starting with a solid vehicle structure.
In either case, you're probably going to need at least several months of construction before setting out.
Before doing much of anything, I would strongly suggest renting an RV for a week and seeing how RV travel works out for you in practice. It will also give you a better idea of some things to look for or avoid in units you're considering.
