The key to making a decision will come down to understanding the outcome vs cost. Typically, a collector will go through these considerations before making a purchase and not in hindsight.
Be that as it may; If the mei is shoshin, then a polish could be rationalized depending. A new polish would mean having a new shirasaya and habaki made, and that is the proper way to store a newly polished blade. You can have the saya maker make a tsunagi for the mounts.
Being suriage this sword may only achieve hozon origami, both of which will affect resale value, a consideration.
So, it may come down to what this sword means to you personally in deciding how much you're willing to spend on restoration.
Regardless, if you keep it as is keep it oiled and keep a close watch on it to make certain the rust does not progress.