Something I have not seen in previous responses that you should consider first.
How old are you and where are you living?
If you are still in your 20's, what is the likelyhood that you will be living in the same play 5-10 years from now. Are you older and plan on staying where you are for the rest of your life? These days people seem to move around and alot, either due to job changes or moving up to bigger homes or smaller homes as the economy effects them.
When my wife and I were married back in '79, we had a small place with no room for a large building. So a 8'x12' shed was the "shop" for many years. We knew as we got older we would eventually move into a bigger place with a decent piece of land for building a shop. Four years ago we made the move into the 'family" home that I inherited, with the assumption that this would be the retirement home.
So when I planned for the shop, I figured on what tools I would be buying over the next 20-30 years to fill it up, used the Grizzly shop layout tool and came up with a 24'x40', which we bumped up to a 24'x50' (1200SF) to add space for a toilet and wood storage. I have 10'-2" clear and heating and A/C. And it is full of (large) tools and wood already. The Ceiling height allows me to not have to worry about lifting and flipping a full 4'x8' sheet of plywood, I never have to look up to see if I am going to hit something. The toilet to great to have, no runner into the house with dirty shoes,when you have to go. The A/C and Heating makes the shop comfortable year round.
I know that if I was in my twenty/thirtys, would not have put such an effort into such a large building unless it was going to be for near full time business use or I was positive I was going to stay there until death.
Large shop buildings are nice, but there are an effort and expense to building and filling them up and maintaining them. (In a single car garage, how many wall shelves and cabinets can you put in it and fit in tools? Now think of a shop with 150 lf of clear wall space for building cabinets. How much is the plywood alone going to cost to build them?)
So if you are young and unless you have $50-100K worth of disposable income you want to sink into something that you may end up leaving in 5-10 years, I would building something to suite your needs now with a little extra space.
You can always design the building so that it can easily be expanded in the future based on how you orient the building on your land.
But if you hit the lottery or have a high income with lots of disposable income, build big and build often..
So to answer your specific questions: How big is your shop
(1200sf) and what equipment do you have...that takes up decent space
(18 large floor model tools) , not like routers and hand held sanders and such. What would you add/change if money and space to expand was no issue
(nothing, I am happy with what I have, I just need to get it finished) ? Any help is greatly appreciated.
Rob