Treadmill help!

I've been saving up for a treadmill for a little while now. So far I've not actually got a specific one in mind, so I was wondering if you guys could help me out! I'm a bit of a newbie at shopping for this kind of thing, so are there any things I should look out for when buying one?

Recently I also looked on eBay, they have ones much cheaper than most places which is great, but what are these things like on the electricity bill etc? lol! Are there any that run on different things?

Anyone help would be lovely

Thanks xx