I'd probably go for an IBM / Lenovo Thinkpad, or a Dell Latitude. There seem to be several on ebay at the moment.
Acer are reasonably well regarded.
Brands to avoid are Toshiba and Fujitsu, as they have not got a good press for quality and reliability these days.
Sony are nice but expensive.
Use the search facility in ebay to narrow down the selection.
A memory upgrade from 512 - 1Gb will cost £25-50, depending on the configuration of memory already installed. Use the Crucial.com/UK online configurator.
SD