Renting them out might need change of use and have tax implications.
Exactly! Change of use is easy to get because the buildings are there.
Why be frightened of tax implications? You only pay tax on money you've made. The only legal way to pay no tax is to have no worthwhile income. Which is preferable? Someone has to contribute to running the country!
Seriously though - renting out buildings for storage, or small businesses (they can be quite low impact) is far more profitable than farming. I get £700/month for a 60' x 30' modern building with light and power and a toilet (connected to unofficial septic tank) What's not to like about that - even after tax?
If the storage or business is low key no one will even know you're doing it. We had an inspection from the council enforcement officer who suspected something none agricultural was afoot, so put a baler and some other machinery in the building, and said the rest of the stuff was all mine - after all farm buildings are notoriously used for storing all sorts. She left quite happy with the situation. However I did apply for retrospective change of use to industrial use for another building, as my son runs a fairly obvious business out of it. We got the change of use with no problem.
If you can get 20 acres with massive buildings for a reasonable price macgro7 then
go for it - it's the way ahead! You'll have enough useable land to feed your smallholder desires, ample buildings to fulfill any smallholders dreams, and sufficient buildings left over to give you a decent income that you'll never get from smallholding alone.
I wouldn't totally devalue a decent building by removing the roof! If a walled garden is an essential extra, then build one! there's no profit in it compared to what you can get from the building as it is.
I also would not include anything to do with rich ladies and their horse in your plans. May be profitable initially, but how much do you value a quiet life and freedom from bitchiness/hassle?