The GPU of choice will depend on the altcoin that you would mine. If prices is not a concern I would definitely suggest the GTX 1080 or a AMD RX Vega 56, but I would highly doubt that you can get it below $500 as most prices of both GPUs is around $700. For now budget wise I can recommend the GTX 1060 6gb since it is priced well in the GPU market with a good performance, the issue her is that the GPU memory which will affect hash rate as some algorithm prefers different memory depending on the manufacturer.
You mean algorithm right. All the time you will not find the difficulty on each crypto. It will changes as the price bumps and dumps in the market. This is happening because of the miners who are been part of mining on various coins mate.
Moreover when you want to buy the mining card, you please buy the card which can able to give more hashes and that card is from Nvidia brand. Then it is much to invest your fund over there. Else you cannot all these thing you please buy and hold the coins for more years.
