That's not an issue. Provided you were previously working in
agriculture or forestry as your main or only job, you continue to
qualify for the condition even if you have to stop working for health
reasons (or you lose your job or you retire). You only cease to
qualify if and when you subsequently start working in a different
industry as your sole or main job. So inability to work due to
disability acquired after legitimately occupying the property would
not have any effect on the qualification.
As for the minimum conditions necessary to comply, there are none. The
condition is a duck test; if it appears to a court that you are (or
most recently were) working in agriculture or forestry as your sole or
main occupation, then you qualify. If it doesn't, then you don't. If
you are an edge case and you think you might just be on the qualifying
side of the dividing line, then you are perfectly entitled to suck it
and see if the local authority (and subsequently a court, if
necessary) agrees with you. But they can only decide on actual,
current circumstances, which have to be in place before you request
the ruling. It's not the local authority's role, or the court's, to
tell you in advance how they will interpret any particular
hypothetical scenario.
Mark