I’m no expert with concrete but an inch sounds a bit thin, normally you would use 50mm for a floor screed otherwise you risk it cracking. If the floor is just a bit rough and dusty you might be better off with self levelling compound. I’d try to do the floor on one go, perhaps hire a van for a weekend and store your stuff in there while the flooring dries. That’s another bonus for self levelling compound in that it would dry relatively quickly. Alternatively you could use an epoxy floor finish which would be dry even quicker at the expense of being more expensive I expect.

now the roof is leak free, i'm looking to attack the floor at some point soon.  The current floor has no pdc and varies between 1.5 and 2 inch thick where edges can be seen.  It might have a small light (800kg) car in there, but mostly is going to be used for woodworking.  No huge kit, just a half decent table saw.  
Would it be best to pour concrete with a water proofing additive onto what is there, or put a plastic sheet down under it?  I'm presuming that plastic sheet will be better damp proofing, but it might be preferable to have the 2 layers of concrete sticking to each other?