I would rather have welded than bolt up. I have several on my property. I strongly recommend spray foam insulation. In my shop you can’t see any exposed metal on the inside not the sheets or the structural members it is all covered with foam.
Go bigger than you think you will ever need and taller as well. I would say a minimum of 14’ sidewalls and preferably 16’. Your door openings will be 2’ less than the sidewall height. You never know what the future holds. RVs, boats with tops, etc are tall. Go with as many overhead insulated doors as possible. I see a lot of shops that are decently sized but have one door and people are constantly shuffling things around to get things in and out. My shop has three 12’ wide doors across the front. I started out with one shop a 50x50 and now added on four times over the past 11 years. I wish I would have just built something much larger to begin with.