If bitcoin goes up to 40k and you haven't bought in, how will you feel?
If bitcoin goes to 0 and you have put in too much, how will you feel?
Nice said If bitcoin goes to 0 and you have put in too much, how will you feel?
If you are not a whale - With a high probability you will not be able to guess the bottom and buy BTC at this point.
You should set several orders to buy bitcoin at different levels.