Yes, I agree with those saying to put in a proper concrete floor first. You will never be happy if you don't. I built an airplane hangar, and could not afford the concrete floor at the time. It worked, but was never a good outcome. The only thing I did right for the floor. Was to dig out a bowl shape within the hangar, and drain the lowest center area of the bowl with big O, then fill the whole thing with 1" crushed stone. That did work. I later poured a 6" pad big enough for the plane to sit on, and that worked, as it had a few feet of crushed stone to sit on.
If freezing in your area is likely, you really need to insulate under the floor, whether concrete or dirt/gravel. If you don't, and it freezes underneath, it'll heave, and you'll be extra unhappy. A few feet deep of crushed stone counts as insulation or, use foam insulation. The non gravel dirt floor side bay of my hangar heaved every winter, 6" or so. It was a dirt floor, so not much harm from that, but not nice either. Remember that if your winter would normally have six inches of snow, and a few months of freezing the roof keeps that snow away, so it does not insulate the floor, and it heaves.
I also tried used rubber conveyor belt. It works, but constantly bunches up, so is never flat, and will catch under a blade, and bunch if you steer over it.
Pour the concrete floor - if your barn lasts you 25 years, is it worth $500 for each of those years to never have a worry about the floor?