Hello, welcome to SRP!

Which stone to recommend depends on what kind of honing you want to do. If you just want to touch up a razor which has been shave-ready but has started to lose its edge, then a high-grit stone is all you need. There are plenty of Welsh slates around at reasonable prices, and barber hones appear on ebay fairly often.

However, those stones will be too fine/slow to put a bevel on a razor which is totally blunt. If you're getting a cheap ebay razor to practice on, you'll need something coarser. The experienced honers on here would probably recommend a Norton 4k/8k for general sharpening, and any of several 1k stones for the bevel-setting. Nortons aren't sold over here, though (please correct me if I'm wrong, anyone) and shipping would add a fair bit to the cost.

I started with a King waterstone with a 1k side and a 6k side, and found it perfectly good for bevel setting and sharpening, and use a Welsh slate for finishing. The King stones are available in the UK for around £30, and you could do worse if you want to learn to hone. As I said though, unnecessary if you just want to touch up a blade which is already nearly shave ready.