well, if you are wanting to add-on at OKW, then more than likely you'll have to go the resale route.
unless by some miracle the waitlist for adding-on at OKW through Disney direct happens to be short.
if you are going to buy resale, and you want OKW, then it will be difficult to find exactly what you want if you are limited to one Use Year.
i believe it's not too bad to have multiple Use Years. just some extra banking windows to juggle. and sometimes when you call MS you have to give them an extra Membership#.
but if you're going to buy a different Use Year anyway, i would stick with the same resort. i think it gets a lot more complicated if you have BOTH a different Use Year and a different resort.
good luck!
(btw, i think the answer is yes to whether someone w/ less than 150 to start can add-on direct through Disney. although i didn't quite test that myself as i started w/ 100 at OKW from resale, but then i added another 100 at BWV through Disney which makes the total higher than 150.)