Have a look for a Solartron 7150 / 7150 Plus. 6.5 digits and ~0.02% basic DCV accuracy.
They go cheap because they have a common issue with the mains filter X-cap failing, this is easy to fix by replacing the mains filter block at the back. I got mine for £60.
The instruments will work with a bad mains X-cap but there will be additional noise produced which may affect the measurements or other test equipment.
They're pretty well built aside from the flaw with the mains filter (probably another RIFA cap gone bad...)