Yes, 300€ would be a nice price if the LSD is in good condition.
But I don't think I would find one like that

Really, when driving moderately, and fast driving and cornering at higher speeds, you can't really see any dissadvantages of the open diff.
Only while taking the sharp turn (on lower speeds, of course) when you press it, sometimes you get the dreaded inner wheel spin (instead of controlable back slide) -> I really hate this.
Also, bad surfaces (gravel, grass, snow, wet tarmac) - there is quite a lot of wheel spin.
I hate the fact that I can't be sure will it start spining the inner wheel, or the back will start to slide when I boot it (I want it to drift when pressed enough).
I'm affraid that if i weld the diff, it will be hard to use it as normal car (understeer in corners when driving like a normal person is my biggest worry).
To put it short -> I don't want to make it a 'drift car', but want it to be 'driftable'.