I have a Rouge H box Kelly from 1980 in perfect condition, and one from 1952 that's still holding up really well (no cracks, no loose stitches). I don't know that you have to ask how the bag will age as it's already 36! The question is - is a box calf sellier Kelly for you? Read up on box calf and understand that it goes basically one of two ways - if you cannot stand marks and scratches you will have to baby this bag a LOT - if you can handle natural handling marks, scratches, etc., that blend into a patina, you're fine.
To me there is little in the world of H, or bags, that is more elegant than a sellier Kelly in box calf, particularly in rich colors like Rouge H, noir, Vert Foncé, with GHW. The tricolor box Kellys are one of my favorite production styles Hermès has ever done. You get a tricolor - which you cannot even order right now - there is usually beautiful contrast stitch - and the combinations are lovely. I would have bought one long ago if they were not usually 32s which are large on me. The price you are looking at is reasonably in line with market assuming the bag is in very good shape, especially since it has what looks to be its original strap.
If you love the bag, can handle box calf, have had it authenticated, and it's in great shape...go for it! These bags are stunning.