A second hand i5 HP ProBook with 4GB RAM. £126. Add to this a 120GB Samsung EVO SSD for £55 from Amazon, and a £6 wifi card from eBay. Less than £200 all in.

Everything works out of the box on this laptop except the wifi, which you have to replace (or use a USB adapter).

I configured one last week. Mavericks runs perfectly. Quartz Express, Sleep/Wake, all works.

http://www.scctrade.co.uk/prod/hp-probook-6360b-core-i5-23gh...

http://www.ebay.co.uk/itm/DELL-DW1520-CN-0KVCX1-LAPTOP-WIREL...

http://www.amazon.co.uk/Samsung-120GB-Basic-Solid-State/dp/B...

I bought a used Dell Latitude E6520 because it was the only laptop I could find with a reasonable price that had a good full HD screen.

As a bonus, it has an i7.

I didn't want/need discrete graphics because hackbooks don't have discrete graphics support (yet?). The only thing I HAD to replace was the WiFi, but I upped the RAM to 8 GB. Other people have upgraded to 16 GB with no problems.