Sorry if my thoughts/ideas are very general but I think each person's shop is unique to their particular wants/needs.
Being 20x25', space will need to be conserved.
As an interim (or permanent) solution, considering making as many of the larger tools (lathe, welder...) portable. Put wheels on or have a cart for moving. These can then be stored under a workbench or set of shelves and only pulled out when needed. In my shop (16'x27'), there are no permanently mounted/located tools. I pull them out only when needed. The perimeter of the area has work benches, shelves/storage, all the tools, fridge... but I keep the main area wide open. When I'm not working on anything, I still have room to pull a full sized pickup truck inside.
For working on the ATV and go-cart, you may want to consider getting one of those ATV lifts. If you have a shorter stool on wheels, you'll be at the right height to work right in front instead of being bent over or sitting on the floor. A small tool cart on wheels may also be handy so you can keep the tools/parts near you when working. A temporary cleanup (if you're done for a day or week or whatever) is much easier since you can just wheel the cart out of the way instead of picking up all the tools, parts and stuff off the workbench or (if you're like me), the floor.
What's the floor? Concrete? I see you're in Ireland so I'll bet you get a bit of cold/damp weather as well. I covered my entire workshop floor (except under the cabinets, toolboxes...) with cow pads. These are the 1 inch thick 4'x6' rubber mats that are put under cows and horses. They're great. They're softer on the knees or your butt or however you sit when working on the floor. If you drop things, they're less likely to get damages as opposed to hitting concrete. They're also a nice insulator for the floor in the winter. When it goes below freezing, I find concrete gets my feet cold. I then just stay on these cow mats and it's much better.
</font><font color="blue" class="small">( All of ur ideas will be included and thanks for your reply)</font> Now that's dangerous... You'll include ALL OF OUR IDEAS? If that were true, I'm sure you'll get some really off-the-wall ideas that would make your shop really interesting.
Brian