Rinouchka gives good advice about making sure the wheels work well. I bought a decent mommas and poppas single buggy for £25 only to find one of the wheels was wobbly. Thankfully, a family member bought two replacements on eBay and fixed it for £13 so still a bargain really, it wouldn't have been without an expert on hand though.
I've got another 'keen'... Ouch!