Here's a good tip:
If you were buying a used car, the common wisdom is to take it to your mechanic for an evaluation. The idea being that you spend a few bucks in advance to preclude spending major bucks later. Now, you can't drop a piano off at your local technician's, but most of them are willing to be hired to make an evaluation house call. Believe me, unless you are talking about a REALLY cheap piano, it's definitely worth it to hire a pro to inspect it.
Now, admittedly, there is no substitute for a good piano, especially if you are a classical player. But, if you are looking for a cheap piano, you may get more value by going with a digital one. Aside from taking up less space and being more portable, it's other advantages include the fact that you can practice with headphones (and not disturb the rest of the house) and that they never need tuning (or much other maintenance). A few years ago, I wouldn't have made the same recommendation, but the universe of digital pianos has progressed quite a bit.
If you are trying to learn to play "piano" then you will want one with weighted keys, and most likely a full-sized keyboard. The best sampled grand sound and action available today, in my opinion, is
the Yamaha S-90 triple strike grand. Unlike almost all others, it samples three different velocity levels per note. (Others use only two.) This allows for more responsive and expressive playing. And the action is the best I've tried in a keyboard.
I can attest to it's superiority over the Kurzweil, at least in my opinion, as I have
a PC2x mounted right above the S-90 and have A-B'd them extensively. One noticeable thing is how much quieter the action is in the S90 over the Kurzweil- which clicks like crazy by comparison. By the way, I also have
a Steinway B (1897), so I have an excellent baseline standard for what a piano should feel and sound like. Everyone that's recorded in my studio with the S-90 has been mucho impressed.
The other advantage of buying something like the S-90 is you get hundreds of other phat sounds besides the piano, and dozens of incredibly hip grooves that are inspiring and fun to practise with.
Last i looked, they were $1899 at Guitar Center, maybe cheaper elsewhere.