Gary
My holy grail is a 40" bed M300, it perhaps says something if I say I have been looking for one at a sane price for about a year now, and I haven't succeeded. Most of what I see advertised appears to be hammered.
Condition always should stand head and shoulders over any other criteria.
History is very relevant – if it is 30 years old but has had a gentle life in a prototyping environment, or school where it doesn't get used much, or home workshop background, this will always be preferable to a ten year old machine that has been used in a time pressured production environment and had the hell bashed out of it.
Your pricing is about right – the only recent M300 I have seen that might fit my bill was £5,900 + vat.
I echo Rik's comments above, I would say personal inspection is vital.
Whilst looking for an M300, I bought a Boxford 280.
Allegedly designed by the same person as the M300, the X10 series Boxford's (10.20, 10.30, 280, 330 etc) are very much like a cloned 90% copy of an M250/M300 – virtually identical in design and capability, just a bit lighter built.
Back in June, a very lightly used Boxford 330 came up (ex school, as most of the X10's are/were sold into education), I bought it via the man that installed it in the school in the first place, and serviced it every year, and I traded up to this.
I got lucky with this 330 – it genuinely is very lightly used, and I paid just £2,200 for it (a picture is in my album) – compare this to the £7,000 + I would have to pay for a M300 in similar condition.
No better X10 has come up in a year, and I just happened to be in the right place in the right time.
The X10's are oddly unpopular, for no good reason. This makes it a buyers market. Several of us on here have got them, and they are very capable lathes. There are several on eBay at £2,500 to £2,950, and they have been there since God was a boy, and will never sell at those prices.
If you are hankering for an M300, look seriously at the Boxford X10 series. I would say that now I have my current Boxford 330, I have given up the search for an M300. I would have to pay £5,000 more, for something that probably wouldn't be any better for my needs.
I hope this helps.
Edited to add a photo of the 330:

Edited By David Standing 1 on 09/11/2017 17:22:59