It still is a little bit "unreal" to me too - I get home from work and bang there it is - "thank god I am not dreaming!"
Haha - but I bet you love it!

SA, I kind of wish I'd never mentioned the hoist now

, but I think your 150 slab should be up to the job. We put our silo's on a 4 x 4 x 150 slab with F53 (I think that's a WA only cheapy), and they weigh around 60 ton pushing on a 50mm wide 3.6m ring around the edge. Never seen one fall over yet!
Don't grano workers put joint lines in a wet slab anymore

I agree with Moe, it is well worth sealing the slab. But I fully understand your problem with curing and moving stuff in - I still have 60 litres of paint that
should have gone on my garage floor. It came from Cementaid (there is a branch in Adelaide), and completely seals it to Health Dept. specs. It's a lacquer that soaks in and can't be chipped or removed, impervious to oil, and can be coloured any way you want. Not cheap, but I think it's one of those things that really makes a good shed a great shed.
Maddog - C Sections were used for the wall purlins - and although I started out seeking a 4 post hoist - the budget is getting a good old squeeze by the electrician and large rainwater tank so I suspect I will only be able to afford a two post. 
I think the 2 post would be better anyway, wouldn't it? Less room when you're not using it! I like those clearfloor ones. But if there is one advantage in where I live, it's that you can build what ever you want (no need for council plans), plumb it yourself (that's legal too), and if I was really crazy, wire it yourself (but that would be illegal

).
got any plans for those tucked away Maddog??? 
You bet! Very simple to make too. I'll PM them to you.
Cheers, Maddog.