I love Jaylen Brown (look at my sig - it was changed on draft night) but you have to give the nod to the 2017 Brooklyn pick. The Cs are guaranteed a top 4 pick in this draft and all 4 are more highly rated than Brown coming out of college. Thus the 2017 pick is more valuable.
As for build or buy, I don't think we've crossed that bridge. In fact, I don't think we're going to need to cross it to a large extent. Why? Look at what you said, Horford is 31. IT wants to back the Brinks truck up in 2018. Bradley is an impending FA in 2018.
We're not going to keep these guys forever. But we don't need to. Boston can be a good FA destination for players who want to win now. Then we're backfilling with Brown, Smart, Zizic, Yabu, 2017 Nets top 4, 2018 Nets (realistically top 6), Memphis 2019, etc. The pipeline of talent keeps coming. So as one drops off, another one steps up.
So take your path #1 as an example and lets' assume Fultz is the pick just for giggles. It doesn't require the Cs to commit to either direction but allows them to continue to explore both. After 2018 season, Bradley is probably gone. But for that 2017-2018 season the Cs field a team that is - on paper - one of the best in the league and could win a title.
After 2018, the Cs pay up on IT unless Fultz shows he can excel. We'd have two more years on Brown before he'd be a RFA, 3 years on Yabu, Fultz pick and Zizic. Those guys start to come up as Horford drops off. Fultz and the 2018 pick come up for contracts as IT or Hayward starts to drop off. Older guys and guys out of contract are continually replaced by the younger guys.
As for redundancy, I think we've seen that Stevens can fit guys into the system outside of the traditionally defined roles. Of that I'm not really concerned. The NBA game has changed enough that non traditional systems can win. Case in point is Golden State - their top 6 guys in minutes played this year is a PG/SG combo, a SG, 3 SFs and a PF (Curry, Klay, Durant, Iguodola, Barnes and Green). In short, I'm not as concerned about redundancy. We've got the right coach for that.