You should always buy the best piano you can afford because you will play better on an instrument that responds with the best sound and touch from your input.
As you get better, the piano will be more likely to keep up with your increasing skills. There is nothing more frustrating than having the piano not perform to your abilities at any level. You know what you want but the instrument can't do it.
This depends on what the piano is used for and your personal needs. For just practicing a good upright would be okay.
For a bigger sound and advanced keying techniques, a grand would be better. A good knowledgeable piano salesperson should work with you to pick out the best choice for your needs.
Always buy from the store that has the most knowledgeable staff, both in sales and the tech people. Those that help you select a piano not sell just try to sell you one. A store with many good reviews is a great indication of quality and customer satisfaction.
Do not be blinded by the most recognized names. There are many superior pianos out there that you have never heard of.
Allow a fair amount of open-mindedness and select a piano by how it works for your budget and needs after listening to all the options available to you. Here is a good tip.
Do not consider any piano that they can't show you a picture of the factory with that piano’s name on the front.
Sounds funny but here is why. Many pianos, even those with old somewhat recognizable names are just generic instruments with no real company behind them. They are called "stencil pianos" made in a low bid random factory, and alike except for the stenciled name on the key cover. These can be a bad choice for many reasons.