Given the size of your league I wouldn't expect your bench to have too many consistent or reliable scorers. Ideally, all your bench players would be players you'd feel confident throwing out there to give you an added boost.
Since you've got a team that's competing you'll want to have your bench stocked with players who can contribute now and give you a boost in your H2H matchups, especially come playoff time.
Obviously loading up to win has it`s costs but ultimately winning is the goal so I`d bite the bullet down the road if it gave me the chance to win right now.
I sympathize with your sentiments about keeping those players to replace guys close to retirement but most of those N/A players are a few years from being viable fantasy options. I`d focus on loading up now, to go for the win and then deal with rebuilding your team when some of your older group retire. There are going to be new N/A players available every year so when you find yourself no longer in contention, that`s the time to fill your bench with those guys.