Agree on rebuilding the line via the draft. Any tackle that is above average gets a bunch of money tossed at him and even the guys who aren't decent still get paid so it seems like a lot of money to throw at the position to be a back-up. That is one place I'd be fine with just drafting a quality guy in the first few rounds. Need someone who can play OT besides Tretter - and even Tretter is probably just a stop-gap, not a starter.
Agree you can get by with Taylor/Walker/Rotterham on the inside as depth. Only the team truly knows what the story is on Sitton and Lang regarding injuries. If there is any thought that one of them may not be back after 2016, then I probably also draft another RT/OG type guy in rds 4-5. There is enough money under the CAP to keep them all in 2017, but that may not totally be the team's choice, so I'd plan to have guys ready and waiting.
However, I think addressing the O-Line in general may not be the right approach. Tretter seems to be pretty good and versatile, but is particularly effective where he was drafted to be - hence, center isn't an issue with Linsley and him. The real problem with the depth at guard was that they were forced into duty at tackle - with Taylor/Walker/Rotheram, there's probably a decent backup there at guard. Tackle is the big problem - Bak is OK, but an improvement there would give us super depth (meaning a new starter with Bak on the bench). And, of course, there are limits to how far any line can be stretched before breaking - this year was pretty horrendous in terms of availability.