Just how big is the slab you want to do? Generally they are only 4" thick, but you use heavy duty re-bar.
Why do you need the floor dead flat? You can't have a slight fall to assist in water run off if it was ever to happen, even if it's only very slight.
Large slabs are hard to make without cracking, but someone that knows what they are doing can do it. Just ensure you use expansion joints around any posts or other un-movable parts. It needs room to expand and contract, even if it's only small.
As for a coating, if you get it machine trowled, they can throw some cement powder down to make a mirror like finish, it is super hard and smooth, but if it gets wet, it goes like ice, really slippery.
You really need to talk to someone that does concrete for a living to work it all out, they will have some good advise for you, i'd make sure i spoke to someone that has been doing it for a while. The younger ones may not have the experience to know just what will last.
When you get it done, do take piccies, we all love good 'workshop porn'