minimum / ideal amount of BTC or ETH to start trading?
the ideal amount will depend on your goal. but this infographic regarding the budget rule might help you to consider your trading budget.
as on the infographic said, all those points may need to be considered. The exact percentages will depend on your monthly money.
the minimum amount to start trading?
again, it depends on the exchange market. normally, spot trading starts from $10 and contract trading starts from $5.
in my country, the local exchanges here allows us to trade as low as $0.7.
By this infographic, that means I am making 6x more money than I am making right now. I can't even cover all my needs with 100% of my salary time to time, of course there are times when I can cover it, most of the times I can, but I have never had one month in my life when I made enough money to pay for housing, foods, utilities, health care and insurance with "just" 50% of my salary, that never happened, it is always around 80% of my salary.
Sometimes 20% goes to savings, sometimes there is no 20% because I need to spend it on something urgent and a must, and yes sometimes 20% goes to savings, but I never had enough money to follow this chart in any point of my life and I have been working for over 10 years, and I am not even making tiny versus people in my nation, I make 2x of the minimum salary here. So, this infographic couldn't be any wrong to be quite honest.