What modules are in piles outside of your current racks?
What bands are going to want to record using your eurorack modules?
While using a rack intended for 19" rack gear is an odd idea, I actually used to have a customized eurorack setup in one of mine. It worked well like that until I outgrew it. As my modular grew, I realized quickly that being confined to 19" in width wasn't ergonomic for anything over 5 minutes of noodling because I had to stand as well as bend down to reach anything not at eye-level.
I have three 19" server racks in my studio that are stuffed with my rackmount gear. They are single-sided (the posts are centered on the bases) and I simply added casters to them from Home Depot so they can easily roll when necesarry. Server racks are easy to come by in the $50 - $100 ballpark on OfferUp or whatever. I used to have the four post type (like the one you are talking about), but found it's footprint to be far to big and using both the front and back was totally unpractical.
Of course, we all have different needs and ideas. I hope this helps!
over:under