Think about the sort of work you're actually going to be doing there. If it's primarily woodworking and metalworking, then separate spaces for those activities is worth considering. Especially if you're welding, cutting and grinding metal, you don't want hot sparks hitting piles of sawdust and shavings. More ceiling height is always good; with the same footprint you can put more stuff in there. Ask someone who installs rollup doors what the standard sizes are and how much clearance they need above; a couple of feet should probably do it, but it's good to check. Think about the tallest thing you'll want to roll in there - an RV or a semi truck loaded with equipment.? Plan for dust collection for the wood shop, and ventilation for the metal shop. Think about where the electricity will need to be to service the equipment you've got (and eventually will get). Can you get 3-phase? That would open up a range of cheap pre-owned equipment most people can't use. How are you going to heat it in the winter or cool it in the summer? The bigger the building, the more that will cost. I doubt that a pit will increase its resale value, but if you really need it, it's worth considering (at least if your water table's not too high)...