Please listen to posters like @Anoni who are giving you business advice.
Start from what profit can I realistically make when going through the options.
You need to be totally cold bloodied about this and do it with a boring bank manger and accountant. Take into account every possible cost and then double it.
That will include making the building fit for purpose, things like car parking, insurance, staff, taxes, marketing, your time.
The your time bit is important. Have you run a business before? Are you happy to make no or little profit for a couple of years, let alone a salary.
You might be able to get grants/funding depending on use.
And it could easily take up nearly all of your time - I'm talking day and night, every day. Are you prepared for that?
This will probably sound like I'm passing in your parade. I'm not, but a few people on mumsnet saying go for it does not a viable business plan make. I know you know that by the way.
Ideally, minimum effort for maximum return at the lowest possible risk.
Doesn't sound very sexy does it? Nor does breaking your heart, suffering from massive stress and losing lots of money.
I do have relevant experience here, both in making investment decisions for existing business and in start ups.
Use the financials to narrow the list of realistic options then do a real business plan and proper target market, competitive research. N.b. Not mumsnet!
If you can rent it out as it is, with few additional costs/risk it might be worth doing so for a year or two to give you some time and cash to invest in other more ambitious and risky plans.
I sound like a right laugh, don't I? Does the barn cost anything at the moment? Maintenance etc. Is is depreciating in value?- err likely to fall down?
Remember, just because you can, doesn't mean you should.
I'm a joy to work with, but it's my job to point out all that could go wrong so you eliminate th risk and spend money, passion and energy on things that will mostly likely work. Or at least won't bankrupt up.
Cheery 
sorry!