Jason,
What you have to be careful of is losing space. Unless you put the 75mm Kingspan between the uprights, you would automatically lose 6" on the width and length of your shop. I personally, because you have a vapour barrier already in, would plasterboard over the uprights and fill the gaps behind with Rockwool. Doing it that way would be a lot cheaper and also you would only lose about 1" in length and width, and the shop would be able to 'breathe'.
You have to be very careful when insulating a workshop, it needs to have some sort of ventilation, as even just being in there and breathing can cause major corrosion problems on machinery and tools, and any fossil fuel heaters, paraffin or gas, should be kept well away from the shop. They produce a lot of water vapour. I left my metal door uninsulated, and the very small gap around it keeps my shop bone dry.
My shop is a sectional concrete, and I stuck the plasterboard with built on 1" foam insulation straight onto the walls, after chasing out the back of each panel for electrical service cables, 1 double socket every four feet.
The floor will present a problem if it is just wooden planks. You will have trouble supporting heavy machinery without it going thru the floor. You can get around that by putting sheets of metal down to spread the loads. Also planked floors tend to be draughty, with the wind blowing up your trouser legs. If you are using a concrete floor, no problems. Either a cheap cushionfloor all over, or an industrial grade rubber matting where you will be standing will keep your feet from getting cold. I'm hard, and have no covering on mine, but it makes cleaning up a lot easier. Also, now I have an oil filled radiator in, that is on all the time during winter, I find the floor and shop a nice comfortable t-shirt working temperature.
The stud wall can just be plasterboarded with Rockwool in the cavity.
Not comprehensive, but I hope it helps.
John