Gibbo, sorry not getting here before now but I converted the rear of my garage to a fish house. A few tips for you, although you are doing the job differently.
You have an advantage by starting from scratch
1. When you do the floor, a drain and water supply is indispensible, if you have room fit a sink as well. With these facilities as an integral part of the hut you can do all your water changes, cleaning tanks, equipment etc. without going outside. (you will appreciate it in the winter
)
2. Line the hut with 2" polystyrene. Don't use plasterboard, it rots in the damp atmosphere!
3.I personally use individual heaters as it gives me more control. Do not use a wall heater, use a tube heater controlled by a thermostat (heat rises and this heater sits on the floor) You can also use a computer fan connected to a length of ducting hose to circulate the warm air down from the roof area.
4. Racks can be of wood or steel. I use ex-warehouse steel racking 2 of with uprights at the end only (lots more room for different sized tanks) My racks are 9' long 4'8" high and 2' deep (this 24" size suits the majority of tanks, you can put 24x12x12 or 15's end on. I have larger tanks i.e. 48x24x15 , 24x24x24high and so on) this racking gives me 3 tiers and can hold in excess of 50 2' tanks.
5. you will also need to store water. I use 3 x 45 gallon water butts with all water going through a purifier, hence no need to use chemical additives. You could also use a domestic water tank and mount it as high up as possible in the roof space.
That should be enough to be going on with but if you need more info on air systems, electic ring mains etc. please PM me
Mac.