You are very lucky the thing was built with 2x12's. This is an adequate structure for floor, but keep in mind that wasn't the intended purpose. You'll need to install bridging, without a doubt. The bridging will help take out some of the "bounce", but generally any 2x12 span over about 12' will "bounce", based on my experiences.
It sounds like you have 22' or so of floor, so you'll need to cross bridge a minimum of 2 times, but I'd go 3, or every 5 1/2 feet or so. Bridging is cheap, get an pneumatic staple gun, and staple them in. This will be the first thing to help take the bounce out. If the floor is 1/2" OSB, it won't do, you'll have to remove it. I'd recommend putting down a 5/8" sturdifloor at a minimum, but your codes may call for something different.
I would also recommend installing a 14" microlam across the center of the floor as an additional center support, this would be about 22' long. Support in at least 2 places, but perhaps 3. Tie in if possible with L brackets. This will keep the floor from sagging in the middle, which it will as is.
The real deal is this... code is a minimum standard, like getting a D on a test. Do you want to build an office that would get a D grade?Code won't help you define the floor that you'll like, just meet the minimums.
I don't really believe you'll need a structural engineer on this project, but they might help you go from a D grade to an A, but it wouldn't hurt to solicit their advice, and if there is one thing about your description we misintrepreted, it might be the difference of your building collapsing. i.e. Sometimes garages were built with builtrite sheathing, this probably is not an acceptable sheathing for what is now a 2 story structure.
And finally, depending upon your locale, follow the authorities. If there is something you haven't shared with us, or we can't see, this is too big a project not to get a permit. If you don't get a permit and your neighbors decide you should have, the city won't be nearly as friendly. If this is in a rural area where b.p. are not required, make sure you talk with a structural engineer that can actually look at the job.
Good luck, sounds like you can make this into a real winner.