to be honest I would be coming at it from a different place than you as I'd build it myself and would be looking for something that needs work, but unless you are buying something that has been completely restored you need to factor in for new consumables like tyres, chains and cables etc.
For me through if you are aiming at that sort of money then you should not be looking at the gas framed mass produced variety with lots of steel parts at all, you should be looking at a minimum of a Reynolds 531 (or equivalent Columbus etc.) frame with a decent group and preferably Campag, or decent Shimano or Suntour etc. They are out there and it shouldn't be that hard. That can of course be a Raleigh or Falcon etc. But because of the nostalgia drive the uneducated will always go for the brands they know or remember so really you should be avoiding them to get value for money
For me through if you are aiming at that sort of money then you should not be looking at the gas framed mass produced variety with lots of steel parts at all, you should be looking at a minimum of a Reynolds 531 (or equivalent Columbus etc.) frame with a decent group and preferably Campag, or decent Shimano or Suntour etc. They are out there and it shouldn't be that hard. That can of course be a Raleigh or Falcon etc. But because of the nostalgia drive the uneducated will always go for the brands they know or remember so really you should be avoiding them to get value for money