I don't know if you already have something to start, like a PC with free PCIe slots, but considering the moment of ETH POS, I suggest you to look for Nvidia cards.
You can mine ETH right now and them you will have a better option to mine another coins, Nvidias are better than AMD on another algos.
500w you can run 3x RTX 3070 for example, or 1x RTX 3080 and 1 RTX 3070, there's a lot of great combinations
Right now I don't think it's a good idea to go with AMDs
If you want more info about BTC mining and Asics, it's better to ask in:
https://bitcointalk.org/index.php?board=14.0