I can't add much to what Les has already said about turning willow.
I can say a few things about your workshop, though, as I have taken over a medium size workshop in similar condition.
In the end it all comes down to money. There are essentially four things to look at:
- The walls: A single skin is not great, and that's all I have. I didn't want to spend a huge amount, so left it at that and only painted it white. If you have the money, you could drywall it. There's a type of plasterboard available that already has a 1-2" foam backing, and that should improve things a lot. However, you will then need to skim the entire space and paint it. It's a lot of work, and if done by a plasterer, will probably set you back the bast part of £1000 (for a single garage)
- The windows, if you have any. Mine were completely rotten and needed replacing, so I got some mismeasured windows from a local company, and had triple glazing fitted into them. Three decent size windows, and proper house door, triple-glazed, and fitted for about 1500. Well worth it for sound insulation and thermal insulation.
- The ceiling: If this is your normal bog standard garage, you either have some sort of flat roof and you can see the rafters, or you have a gable space, also with rafters visible. Put insulation in (any DIY store has the stuff), at least 100mm thick, plasterboard to hold it in space, then skimmed and painted. You should be able to get all of this done for about £800.
- The floor: I wouldn't bother doing anything other with the floor than levelling it (self-levelling compound from Screwfix is good) and then giving it several coats of floor paint. I went with epoxy paint, that's the kind of stuff used in car workshops and airport hangars. Very expensive, a mess to put in, but lasts forever. You can always put some rubber matting on top to insulate your feet a little.
My workshop is cold, but I prefer to work in relatively low temperatures. Anything above 5 degrees is doable, and above 10 degrees is comfortable for working. Just make sure you have proper work boots with thick rubber soles, and put an extra T-shirt on, and you'll be fine. My shop is about 30m2 (that's about 270 square foot), and I have a small 2kW electric heater in it. When it's really cold (i.e. like right now, we have about 0-2 degrees Celsius) I run the heater for an hour before I start working, and that's enough.
