For that bike being 'like new' you cant see much from the pictures, but would be checking to see if the seatpost & stem are extended over their max, as both look high, especially the seatpost, putting them back the normal positions, would it fit you? Not seeing any rack mounts on it, you can use P-clips as an alternate.

For the proposed triple conversion, can you get a 6500 LH triple shifter? these have been OOP for 10+ years now, if you could, you would also need a new BB & FD as the double won't be compatible. Amuch easier / probably cheaper option would be to look at a larger range cassette, even if you had to change the RD, it would be cheaper than changing the crank, FD and shifter