The Bills need a LT. I'm sure that they will draft one on first day. Bell has been overmatched and he'll need at least another year or two to solidify his play, if he can. The Bills can't afford to wait and see. I would pencil in Bell as a future backup or possible RT starter, if he develops.
I'd like to see what a healthy Butler can do at RT. He never had a chance due to his injury. He is injured a lot though... I think that Meredith and Scott have enough ability for one of them to be a solid backup.
The starting interior is set. They need more experience playing together. There's no way to know what McKinney can do because he was injured so quickly after coming in. You'd have to figure though, that between McKinney, Simmons and Gaddis, there has to be someone who can backup at center and guard.
So: 1 LT + healthy team + additional year experience = decent OL. The Bills can re-evaluate things and make any final tweaks after next season.
Besides a top pick, I wouldn't blow the Bills wad on the OL. There are too many needs in other locations, like DL...