Hub nut is 36mm as I recall, a Halfords impact socket fits perfectly - it isn't a "deep" one, the key measure is wall thickness. Certainly my CV joint did not knock and shake the whole of the front - it started with a droning vibration, that got worse under acceleration, and got worse accelerating out of left hand bends (right hand CV). It got to the point where the car was hard to drive as I couldn't stick the power down! However, it never "knocked" - it "grumbled loudly".
I would get underneath and set someone to move the steering from lock to lock. There may be a cheapo ebay part that is a bit wrong, or something could be loose. Something daft like the wheel arch liner coming free makes a horrific noise.
Check the joint boots - if the boots are full of water, then yes, the joint is toast.
If it is the CV, the allen bolts are fine - the nuts are locked at the back, so it is simply a question of a key + extension, and undoing them. No need for a spanner behind. Much easier than they look!
On the "value of the car" thing - the only measure is what it would cost you to replace it. Even if you put £200 of parts on a £600 car .... that is still cheaper than buying another £600 car! I got my CVs from the local specialist, probably not the cheapest, but they fitted perfectly which is what you need on Sunday evening when you need the car on Monday....