An original Series 70 Gvmt in pristine condition, UNMODIFIED, with box and paperwork might bring in excess of $1K to the right buyer, who is looking for the real deal for his collection. A NIB, apprently unfired, example sold locally at a local shop for $1100.
That said, your post said that the piece being considered has been modified, (sights, trigger, springs, etc). Depending on the mods, who did them, and what documentation there might be, in my opinion the value has been signficantly discounted unless the original parts are available and the pistol can be restored to a truly original state. With no more info and no pics, I'd hazard a guess of around $800 for a lightly modified S70, with no bubba smithing, no rust, etc.