I don't have enough experience to say which grits to start with, but if you go orbital, try to figure out how big of a project you are creating before you get too far.(and yeah... I would trash the wall in two seconds with my belt sander skills
)
My 0.000002 cent advice is this- That is some really straight grain softwood which will really show sandscratch in any sanding motion that is not parallel with the grain- the orbital sander of course making scratches in a 'circular motion' (cue some frank zappa song from college I can't remember)
If you sand it, go with the finest grit paper you can which will strike a balance between getting the job done and scratching the surface. Test sand an area and test refinish it to make sure you aren't inducing any orbital sandscratch which you might not see on the bare wood, but will show when you finish it. The redwood is soft and the gut instinct is going to be to pick a course grit and blast through it, and you might think you are off scott free, but when you go to finish it, the finish will show the sanding damage You need to have a plan of the finer grits you need to go through (and test it) before you tackle the whole thing so you don't go crazy. You may even decide to not mess with it once you find out how much work will be really involved. I wouldn't have the patience!
Now that I think about it, if I had to do this, I would experiment with a cheap in-line pneumatic sander like they use on automobiles to get the straight sanding stroke parallel to the grain (like a belt sander) but not have the tendency to gouge as much as a belt sander would when it catches an edge. It might be too narrow, but you could apply a softer touch and it has the soft backing without the belt sander's hard edges. Who knows...
Good luck...