Good R4s with little to no work are for sale, but you pay the price.
Project R4s that require real (a lot of) work are cheaper, and the "inaccessible" structural areas will be affected. It just depends on how much work and effort you are willing to put into it.
The so-called inaccessible structural areas are actually accessible as long as you separate the body and chassis.
Read the projects section on this forum to see what you might up against on working on or restoring a R4.
Good luck.