I am not giving you professional advise, repeat, I am not giving you professional advise. It cannot be construed as such.

I am an engineer. First off, I would imagine any structural defects in those hand hewn joists and columns would have shown themselves by now, assuming they are sound/termite free/etc. That's old growth wood, broski. I would also imagine any defects would have been discovered by your (assuming) competent home inspector, considering you just bought the house.

I would not waste the money. You will never even come close to exceeding the live load for that floor. Considering parking garage decks are designed for 40 psf these days, you'll never come close.

You even math, bruh?