Hello! I did a recent post which is closely related, so forgive my laziness in providing the link 🙂
You'll want to unpick the two issues of land type, and status of the building/ruin, as they're slightly separate in how one addresses them.
Please don't rely on what an agent or seller will tell you about what is/isn't possible - even if well intentioned, it may be incorrect. Also, be wary of generalisations about what you might "get away with"! Get a lawyer to make sure, or speak to the local council. The extent to which a land type or building status may be changed is based on legislation, but subject to local interpretation, so what works in one area may not be favoured in another.
Some areas are, however, genuinely delighted to see ruins restored and lived in, so this is not to put you off - but find one or two actual properties you like, and investigate from there. Good luck with it!