A leaking silencer won't be the cause of your emissions fault, although it is a failure by itself. New exhaust is £100 and less than an hours work. Sorting the high lambda number will require some proper diagnosis, but probably won't be too bad.
New cats(I would never buy used ones) aren't particularly expensive, but the costs are adding up.
Brake pipe is a trivial repair, and is common for a twenty year old car. You can't tell how bad a rusty looking steel brake pipe is, so they're listed as failures. Which is what a safety inspection is for. I've replaced several like this, and every one was justified. Smothering them in grease so they can't be examined is a bodge that I think should be a failure all by itself.
The welding is your real problem. It's why I got rid of my car last summer; a critical look at both sills, NS sill end to subframe mount/floor/seatbelt mount, NS wheel arch and quarter panel, front and rear chassis rails meant it wasn't worth fitting the new sills I had in stock. I had already repaired the OS sill end/floor and wheel arch. My car was repairable, it just wasn't worth the time even though it was mecanically strong(recent suspension overhauls, good engine, refreshed interior, everything working correctly). I strongly suspect, because I haven't looked at it, that yours is the same. Most of these cars are at this point in their life where even skilled DIY repairers(like me) won't repair them. Which is why they're not 'classics' - whatever that is - yet. There's still too many of them about in daily use. Give them another 10 years, and the remaining few dozen will be like a Cresta is now. You'll see one at a show and reminisce before moving on to look at something interesting.