Avoid ebay.
In the UK look at:
www.theoldwatchshop.com
www.chronomaster.co.uk
£2500 is a bit low but you should just pick up an old Explorer II without papers for this.
£3000 - £3500 will get you a much nicer selection.
You are right in that over £4000 you can buy new old stock from ADs. John Pass in Crewe is an AD with a good pre-owned selection but you will have to phone them to find out what's in stock.