Hi, I have completed three workshops, 18mm 0SB is fine for floors on a 16 inch joist spacing, go for the 2' x 8' with
T and g edges then you won't need noggins under the joints, someone commented that it didn't look very nice I disagree, if you can get a couple of coats of polyurethane floor varnish on it before you walk all over it too much it will look great.
Loft insolation is good in the walls don't recommend it under the floor as it's useless when wet you need something like King span. Would recommend beef up wall timbers to 4 inch that extra insulation in the walls and at least 12 inches in the roof means that you can keep it to a nice 12 /14 degrees in the winter with an oil filled radiator and it will cost next to nothing to heat.
Somebody mentioned using UPVC window frames, this is a good idea and if you want to save money go and see a window firm and snaffle some of the secondhand stuff they take out, you might even be able to get a door and a frame.
Think about where your bench is going,( I always prefer mine fastened to the walls it doesn't move then ) and put in a horizontal batten at the right height to fasten to later, coach screws are useful for this sort of job.
When you are making the walls build them up flat on the ground it's much easier and then move them into position and coach screw to the floor joists . This sort of project is fun, hope you enjoy it . Ian