A Keyboard is easier to start learning on than a traditional Piano. The keyboard can play backing tracks and you can get far more out of it. But if you have got a piano that's fine as well. the keys are easier to press down. Its more portable
You can spend from £20 to £3000! You can always pick them up second hand from Gumtree and ebay, at car boot sales and charity shops
Both Yamaha and Casio make excellent keyboards. Like everything the more you pay the better it will sound.
The minimum specs for a keyboards should be...
61 FULL SIZE keys (Some of the cheap keyboards have keys that are half size for kids)
Touch sensitive (so that you can play loud and soft)