My answer would depend upon your mindset. I get the impression from your use of the words 'proper' and 'upgrade'(to contractor+ of all things) that you consider the Shopsmith a less than worthy tool for your shop.
I do not think that is the case. Properly adjusted and maintained, it will perform as well as what I think you are considering upgrading to.
Surely 'single purpose tools' have an advantage in that they do not interfere with operations of other single purpose tools.
As for 'return', refurbing is mainly a labor of love. I do not know what you have invested into it, but unless you paid too much, it will get back more if it
is refurbed.
As has already been said, put back in running condition and properly maintained, it will be around for use by your grandchildren.
I dare say any contractor+ tools you purchase will not.
Folks here are using machines that are 50 - 60 years old(as is Shopsmith on the production line).
No they are not using the table saw function. That is one additional tool I would suggest. Either that or upgrade to a 520.
Finally it can serve as a backup for those contractor+ tools when they break down.
