To answer the second question first, pretty damn necessary!
You get a new Propshaft that helps to prevent ball breakage - it wasn't a universal problem, but some early Geos were breaking a lot of paint, and the new Propshaft resolves that issue.
You get Nyloc patched screws that won't shake loose as you fire. It's odd, as the Geo is a pretty smooth marker, but for some reason, even though the marker doesn't feel like it is shaking, it none the less seems to rattle it screws loose.
The Nyloc patched screws won't shake loose.
You also get a new reg piston which has been altered so that the reg seat seal cannot be blown out by air getting under the purge hole in the centre of the seal.
If the Geo in question has never had a G-R2 upgrade kit assigned to it, then it should still be possible to get it sent to you.