Like a proper old frame, 1995 steel Marin.

I love the frame, it won't take bouncy forks, but I'd like to bring it into the world of brakes that work (only probably 140mm ones though).

Doable?

Argos quote 50 quid for labour and you supply the disc mount

Will the frame and forks take the stresses and wotnot though?

a proper frame builder (like Argos) would be able to advise better on that, but you might need it braced.

Frame probably won't, so they add a brace between the stays to strengthen it. Saw an early P7 fold by there a few years after having a disc mount fitted without the brace,the replacement with the brace was still going strong last time I heard.

Or go for Magura hydraulic rim brakes. Obviously not as good as disc but still very nice

+1 vote for the magura rim brakes, would definitely save a lot of faff and they're actually pretty powerful

There's always the drum brake option 🙂

Maggies are a lovely alternative to discs and bearly heavier.

That said, my brother had a disc mount added to the rear of his mnt vision (pre-disc version) by a family friend welder, it's been fine despite the paper-thin alu shell of the swingarm.