No indoor critters. Just the two cats, and they live out in the shop.
I've actually been trying almost that exact idea- find a new or used desk that can be modded or added on to. My current desk is one I've had for years, and have modded extensively- part to rebuild/strengthen it, part to alter it to be more useful.
The problem is, I like the current format- a corner desk where the user sits right at the apex, facing the corner. Not sitting at one wing or the other. And the only desks I can find that have the same format are new and semi-high-end, and I'd rather not blow $500-$700 plus a couple hundred in shipping, especially if I'm just going to end up with fancy pressboard anyway.
I have a vague design for basically this same desk, with lengthened "wings" and a proper welded frame- like you say, out of 1" square tubing. Make the top out of something good but inexpensive, like cabinet plywood, stain and seal to taste.
Honestly, I could kick it out in a week, for maybe a few hundred, I've just been waiting for that free week.