I think it is the way to go, MA. I think it is far more likely to be possible to do this sort of thing than to start from scratch on a piece of land that has no history of residential use. And I think the finances make a lot more sense, too. Not all farm buildings will have an Ag.Tie, but you can find that out from the selling agent, or better, from the local Planning Dept (it's public information, so you have a right to find out about it.).
I think you would need to satisfy them about certain aspects of the development, but I think a plan like this would be considered by them.
Remember that planning decisions are made by a planning committee that meets regularly. The decision is not made by the officers who are employed by the council, but by the elected councillors who sit on the committee. The committee will follow guidelines, and also recommendations put forward by the officers. But this is not set in stone.
If you are serious about it, get others involved, cost it out, find out about how to finance it, develop a proper plan, and then ask to see a planning officer for an informal chat about what may be possible and whether they would support such an application. Also, get the names of the councillors on the planning committee, phone them and ask to run your ideas past them, so that you can gauge their response. They shouldn't be unapproachable - this is what they're for. You will then be in a position to know what sort of property to look for, how much you can afford, and what is likely to be acceptable to the committee.
I imagine the final application would have considerable scope for negotiation - you may have to settle for certain conditions upon the plan before they will approve it.
Ultimately, there is no absolutely certain way to be sure of an outcome, and this means that any sort of plan does carry some risk. But it is possible to apply for PP before you buy the land. Doing this, though, would require the seller to bide with you while the application went through, and if he got a better offer in the mean time, it would all come to nought - at least for that attempt.
I was briefly involved with
The Community Project, while they were still looking for premises, and we looked at several stately homes, farm houses, etc., before the group decided to bid on an old hospital, and convert it into a co-housing project (unfortunately I had to back out of the project at that point). It seems the UK Co-Housing Network is now run from there (same website). I suspect they now have a wealth of information that they could share.
