I definitely second the notion of starting with a mechanically-sound vehicle!
Now, about width.
WARNING: Long technical answer:
"Legal width" is a complicated matter, because there are so many variations of "legal". They moved a complete Space Shuttle on public roads recently -- legally.
Up to 80” (6’ 8”) wide, all you need is normal passenger car equipment and off you go to Grandma’s house.
Over 80” wide, you need additional lights, primarily the short row of three lights in the middle of front and back. This set is called Identification Lights and serves to notify motorists that the vehicle is wider than a regular car. Most people of course have no idea about this, but truckers definitely pay attention to those little groups of three lights on narrow roads in the dark.
Then we get to 102” (8’ 6”). That’s the widest you may be if you want to just get in and drive anywhere and anytime you want. Any wider, and you have to apply in advance for an Oversize Permit from the state Department of Transportation or whatever agency is currently handling this in the state. And you need such a permit for each state you drive thru.
The permit tells you what day you may drive, what roads, and other restrictions. Many cities prohibit Oversize loads during rush hours. Generally, you may drive only in daylight.
When I hauled such loads, the company made the arrangements and I picked up the permits via fax at truck-stops. Things may have changed.
And you must have a big yellow Oversize Load or Wide Load sign on each end, and red flags at the corners.
Next, 10’. Now my knowledge gets a bit rusty, but some states let you run at night up to 10’ wide, provided you install lights on the widest points of the load. (I ran such a load down I-5 once – a gazillion-dollar adapter-ring for a satellite to be launched from a Space Shuttle. Those folks paid quite close attention to how I installed those extra lights and everything else!)
The next step I’m aware of is 14’. That’s the common width of those “manufactured homes” that consist of two or three sections. In this range of 10’ to 14’ you may need a Pilot Car or two – more so on narrower roads. On nasty roads you may even need Highway Patrol escorts, all of which you must pay for at overtime rates.
The “manufactured homes” industry has tried selling 16’ wide single units, but I hear that this proved impractical.
But the darndest huge wide things sometimes need to be moved; usually industrial machinery or ships. Or that Space Shuttle. You don’t even want to think about the logistics for this.
Now we need to get back to a basic point we skipped over earlier. A fundamental rule for getting an Oversize Permit is that it must not be possible to make the load smaller. If there is any reasonable way you can make the load smaller, no permit. (There is an exception for stacks of roof trusses.)
That leaves us with the question of whether the permit-issuing agency would try to judge how necessary your move is. And I have no idea. I did this sort of thing only commercially. Any oversize load is a traffic hazard, so I imagine they might not want “frivolous” hazards on the roads. If anybody could research this….
Oh.... One more thing. You have been talking about vehicles, and I have been talking about cargo. But so far as I know, the same rules apply. There are "heavy-haul" trailers out there that are 10' or more wide. They need a Permit just to drive empty.
As always, I'm not perfect, and neither is my knowledge. So I don't charge much.
Oh, still one more thing. Next time a portable carnival comes to town, study that equipment. Even their biggest Ferris wheels and carousels and haunted houses fold up to 8' 6" wide.