Hi Klunk, my experience is rather old, but the shed / workshop has survived a move and still looks OK after 24 years. I would draw and plan a workshop out on paper to suit you and location bearing in mind the light available and position. You may want to have a view of the house or whatever.
This gives you an idea of what you want and or could have. I always go for the sky rather than start with a compromise.
I was surprised what poor quality garden sheds were, look OK but are drafty to be in with ill fitting doors and low quality glass.
They also come with very flimsy floors, I suggest 3/4 ply 8 x 4 sheets here. I upgraded the floor when my shed was moved.
When you have an idea of your requirements check out the sheds.
I had mine made for me, it works out cheaper than you might expect. (I constructed my first one). I found a wood company in Gerrards Cross who built cheaper than I could buy the wood, and delivered and errected, that was extra.
I would recommend double glazed glass to fit in the wood surrounds, not window units, it is surprisingly cheap.
I would suggest you go with the idea of an "extra room" rather than workshop. If you can think ahead to the fitting out then put battens in to support shelves etc. I insulated mine with some solid foam type material that came free, then I used tongue and groove cladding inside for the finish. As I said before, it was like an extra room and I have never regretted it. You may move house and this will be something you can leave behind.
I have no experience of metal sheds but they may come in as more economical in comparison to above.
There are some " musts" A solid floor and double glazed windows. Also consider the roof, after experience I favour a flat roof sloping from the front with an overhang at the front of about a foot. (You can hang a flower basket from it then). I needed the highish roof so that I could rig a yacht.
You will find kitchen units are very good for constructing a bench, with an "off cut" thick work top as surface, so this can be any length to suit space. We did have a bespoke kitchen furniture company in Watford and they made drawer units for me, low profile drawers are quite cheap. I had their bottom quality and collected them myself and they were very cheap. You will not need the legs they come with for kitchens.
I hope some of the above helpful, this is my experience of mistakes made in the past. I use convection heating and if starting now would fit bright LED lighting. It is a good idea to work out where major tools are to be located and allow for that, as far as size goes it will always be too small after a couple of years!
I have repeated some of what has already been said hope you will forgive me.
Best regards
Roy