Advice on fixing my shed

I've been sharing a shed since I got my lottie with the lady who's got the plot next to mine but its getting a bit crowded in there, so when I was chatting to the secretary of our site I asked about a shed I'd seen that didn't look occupied. Anyway, he said it'd been left behind by a previous plot holder about 2 years ago, so if I got a new lock I could have it Hubby and I went down there yesterday to sort it out. From the outside it looked fine, just a bit tatty, but inside it was a wreck. The walls seem to be held together with odd bits of wood nailed in here and there and the roof is leaking - not really bothered about how it looks in the inside, and the roof just needs new felt on it, but the main worry is the floor - or lack of it. There doesn't seem to be any real floor at all, just the odd scaffold plank here and there, bricks laid randomly and the rest is is is compacted earth, so anything I put in there will probably get really damp. Whats the best way to go about fixing it? I thought about waiting till the weather is better, then digging away at the floor to level it off, laying some membrame down then fixing some scaffold boards down to make a level floor - do you think this would work?