The answers to your questions may be very simple, such as, Start investing with as much as you can afford to invest. If you have less amount of money then invest less amount and if you have more amount of money then invest more amount, Aim to invest consistently.
Many people invest by borrowing money from others. If one has a fixed source of income he can borrow money from others and invest but does not have to sell his investment. Because people don't always have money even if they have a job. So if he invests with a loan and later if he can repay the loan with the money he gets from his job at the end of the month, it means he can easily repay the money he borrowed from others. As a result, he no longer has to sell the investment to repay the loan.
No matter what it is not even advisable to take loan to invest in bitcoin. In bitcoin investment you must earn for you to be able to invest that is why it is an investment that one needs to have a source of income which you do not need to borrow. I think the only reason that would make one to borrow money to invest bitcoin is due to desperation, just to make money quick and when you depend in taking loan to invest bitcoin it is very possible that the investment won't last long. Loan for investment is stressful, it is better to get a source of income , no matter how small it maybe you can afford to invest with the amount you can afford and your investment will keep going smoothly.