Loads of good advice on The Poultry Pages group on Facebook... here and also on Omlet forums here
To answer your questions... I think that is enough space for 3 chickens, I wouldnt lay slabs as my girls love scratching around in the scrub. You can always "dry up" any mud by adding some sand.
Mine dont smell and I poo pick as much as I can. Clean out bedding once a week and a thorough clean/disinfect every fortnight in summer and once every 3-4 weeks in winter... We also have a large plastic flexi rake which DH is obsessive about uses to rake over the run when he closes them up for the night.
Our poo and bedding goes into our compost and well rotted compost doesnt smell. I dont use shavings as these are not good in large quantities in compost and I find they go a bit claggy.
In terms of mental enrichment... there is loads to do... Ours have a dirt bath in a big cat litter tray... which I hide things in and rotate old earth, grass cuttings which they adore rooting through. Fat balls hung from the run roof and also heads of cabbage, spring greens or similar provide hours of entertainment. Can also do this with apples etc. Sometimes I skewer a corn on the cob and stick it in the ground also... In the summer I freeze grapes and put frozen peas or chunks of cucumber in the water so they can pick away at them... like apple bobbing!
Mine get all the old stalks of broccoli, brussell sprout plants when they are past their best in the garden.
Ex Batts are fine, but need a bit of extra TLC - we have bantams which are very very friendly and a bit smaller.
I dont have a chicken first aid guide... touch wood have never needed one... but the Katie Thear book is quite well regarded as a good starter guide Amazon
Happy to answer anything else... just ask!!