I live in VA and my workshop is an unheated, uninsulated garage. I work out there year round, even when temps are below freezing. I can't say it's comfortable in the winter (or when it's 100 degrees with 95% humidity) but I haven't had any major issues. I have had some big box green wood warp on me after I built a couple bookshelves and brought them into the house, but that was more a factor of the green wood than my "shop".
I marked the holes by putting a dowel center in the end of a long piece of 1" dowel. Run it through the holes in the base plate, and bang on its end with a mallet. Rotate it a bit and bang it again, and repeat. Odds are the dowel center won't be precisely in the center of the dowel, so you'll be making a small ring of marks. The center of the hole is, of course, the center of that ring.
@cahudson42 I bored the holes in my sandwich top with a 3/4" power auger bit with the drill in a portable drill guide that kept it vertical and in a way a bit like a plunge router. The guide screw of the auger was dead easy to centre over the laid out grid points. The holes are dead straight and clean sided. The auger bit was new which probably helped. As stated I have three rows from an end vise running the length of the front as well as from the front vise and holes in the front apron to support long pieces as well.
After I left the magazine for a new job, I did all of my woodworking in my small two-car garage, but it was a struggle. Power, lighting, and temperature control were limited. And because I shared space with our cars I was always moving machines around, often while in the middle of a project. On top of that, my tool and machine collection had long since eclipsed the space. For all these reasons, fixing up the garage wasn’t an option. My finished basement is a family room, so it was out, too. An addition to the home or a stand-alone building would have been too expensive. I had to find a better path to a shop.