Do not be misled by the title of this box set. It is not a complete set of recordings of the operas. In fact there are only four complete operas recorded here: Rosenkavalier, Schweigsame Frau, Arabella and Capriccio. Of these at least three are recordings of live performances. Where/how they originated is anybody's guess. They are marketed by a firm calling themselves "The Intense Media". Rosenkavalier does not profess to be recorded live though it could be. I have only listened to this so far. I suppose they could all be described as 'Historic' - the oldest being recorded in 1947.
As might be expected, the quality of the sound leaves something to be desired: certainly not a 'Full Frequency Range Recording'. You will not get great clarity of instrumental sound or immediacy and intimate quality in the recordings. What you do get, at least from Rosenkavalier, are Great ( in the true meaning of the word ) singers, Great characterisations and Great voices. You also get Great conducting from Karl Bohm and excitement and drama - and humour and pathos.
You will not get, of course stereophonic, high fidelity sound or - God be praised - sopranos with flat top notes ( or hardly ) and certainly no wobbling. That may be considered a fault by the modern listening public which delightedly applauds the most inaccurate and ghastly intonation. ( One might think it has no understanding of singing at all were it not for the fact the greatest singers alive at the moment do NOT wobble. How come those aspiring soloists do not take note of this fact?)
So ten pounds for four old recordings? That must be up to you. I think, on the basis of what I have heard so far, that it is a fair deal. I shall be happy to revise my opinion in the light of the remainder of the, as yet, unheard recordings!