So does this mean that trying to get to that vendor as low level as possible is desirable to min/max stats in the long run?
Also, I know you quit gaining stats after level 20, but would the merchant restok at 21, 22, etc?
yes to the first question, i have no idea on the second.
for the first question if you do things normally and dont miss anything you can probably get there around level 8 to 10. if you use sneak/invis and know the game locations for the inert stones you can get there earlier.
for the second question i dont think you can really get passed level 20 without abusing a glitch or a cheat so its kind of irrelevant.