The last two bike I have purchased were second hand and I have picked up 4 in total for my step sons. Ask as much as possible about the history of the bike. There seem to be so many people out there who buy a bike use it once or twice and it then languishes in the back of the shed or garage. As long as it has been stored in relatively dry conditions it should be fine. The last bike I picked up was a Claud Butler legend hybrid. It had been ridden only twice carefully garaged. Ok so it wasn't state of the art but perfectly functional and only cost £50. Given the cost of replacement parts, I would tend to avoid anything that may require substantial rebuilding, unless of course you have a stash of salvaged parts which will do the job.