Whether a piano is good enough depends of how far you wish to go.
If you want it for leisure and occasional playing, getting any piano will probably do. But if you wish to study piano seriously, you need to get a decent piano.
I've been to Cristofori and Asia Piano showrooms to help friends choose 2nd-hand pianos. I'm not an expert, but this is how I would choose...
1. the tonal-characteristics - whether it can give you warmth when you want it, and bright clarity when you want it; this relates to how much "character" a piano has. Some (unbranded) 2nd hand pianos I tried simply don't have enough versatility in this aspect to keep me interested to play it. The sound you get from the piano will give you the motivation to continue learning your pieces. If you get a piano that gives you predictable and "boring" sound, it will most likely end up unused and collecting dust..
2. the key response - whether the keys can react fast enough when you play fast passages. Some badly-maintained old pianos have keys that get stuck or jammed. This will ruin your mood, seriously.
I always believe if you want a child to go far in piano playing skills, you must get that child the best piano you can afford. The piano's "sound" will motivate the child. If you're buying it for study, consider it an investment for development, not as a vase.
Yes, Yamaha's and Kawai's are usually among the best in the showrooms. Although the sales assistants I met at those 2 showrooms were very helpful and knowledgeable, do try to bring a pianist friend along to help you choose.
fyi, some will offer a buy-back deal if you change your mind after your purchase.
