Buy in little by little with perhaps $50 per week until you reach the maximum you feel comfortable with. You will not gain so much, but it will also soften the price drops.
of course these steps also adjust to market conditions that are running, if the price is not allow us to buy, better not forced to buy it
There is no such amount to invest, its depend on ourselves. As long you have your extra money for investment still it will be acceptable. Then if that time you already invested, try to hold it cause the value of bitcoin will goes high which means additional money to your investment.