my $1 (inflation)
As we both have a RF31 flavor machine, and if I were to upgrade the first thing I would look at is envelope. Particularly in the the Y direction. I don't know about you, but this has been the biggest limitation that I have had to deal with. You are looking at a square column machine, so Z has already been addressed.
I would not worry to much about a genuine RF machine as a LOT of the pieces parts are going to the dust bin anyway.
While I like the IH machine, Remember if your going to convert one the problems others have had with IHs service on the kit stuff. If your going to roll your own, then forget what I have said.
Of the ones listed, I think this would be a great choice for a conversion..
Art
AKA Country Bubba (Older Than Dirt)