@HeebieGeebie
Welcome to the expat.com forum, and good luck with your (property) adventures in Bulgaria!
It's all pretty subjective, I'd say, so some will love your idea, others will hate it. Personally, I'm tending to the latter side, so I'm very sorry to be raining on your parade. :-) Hopefully, someone else will give us the bullish case, for comparison.
In any case, Bulgarian property prices are rising, especially in Sofia. So it might work out well, due to appreciation/leverage, despite any concerns.
However, I'm not at all sure about buying something specifically for Airbnb (as you'd hardly use it), as opposed to something to live in. I also wonder about how easy it is to get a Bulgarian mortgage (as you're perhaps not a resident) for this... or using a UK mortgage for an overseas purchase.
And I'd have to think carefully about buying something in joint names, especially if I were the one plunking down the 50k deposit. But if it's her money, then it's a no-brainer. :-)
You also have to consider whether to apply your cash + borrowing power to a UK property, which might get better returns, and be easier to manage as you're living there.
I like Airbnb (I was a Superhost for many years), but I don't think I'd buy something specifically, or principally, to be an AIrbnb let. Especially if I had to borrow money to do it.
The Airbnb hosting fee (as you're not here to manage it) is a big unknown. I still have my apartment in Buenos Aires, and my neighbour's commission for dealing with Airbnb means I make almost zero profit - and that's without any rent/mortgage costs. Most that I see (and my neighbour is no exception) charge based upon your gross revenue (= nightly rate). Even small gross percentages look pretty distressing when you compare it with your net after mortgage payments, property taxes, and general expenses for enabling strangers to live in it.
But there are folks with properties in high-demand, high-price locations that do have mortages and still make money. So I certainly don't say it's impossible.
On the other hand, it seems perfectly understandable that you want to own a place in Bulgaria that you can come to regularly. However, my suggestion would be to explore what you can get here as a small, lock-up-and-leave apartment without needing a mortgage. Sadly, 50k euros no longer goes very far, especially in Sofia. So this means you need either a bigger budget and/or consider other locations.
I bought a studio in Plovdiv for 40k, about 4 years ago. (To rent on Airbnb, ironically, and before you ask how it went, let me say: Pandemic. I bought with cash, so no big deal, but if I'd had a 150k mortgage it would have been much more unpleasant!) Plovdiv is a great city, and we very much enjoy living here... but I doubt a (decent) 50k studio is still possible. There might be smaller cities (e.g. Stara Zagora, Veliko Tarnovo, Sliven, Ruse) where you can. However, you can definitely still find properties under 50k in/near Bansko (main ski resort, and also lovely) and in/near Sunny Beach (main beach resort).