I think 0.1 BTC is the minimum investment. But do not invest all your money for BTC. Because in the far future, BTC has bubble
That's too much. 750$ is a lot for a beginner to play with. 0.04
BTC is ideal. Not to much not to little. BTC isn't a bubble. It does go up and down i waves but it isn't a bubble.
There is no limit and restrictions on the amount of investment but you should look into your self, if you are dipped in more water then you should invest with more amounts but remember that you know well about the market. On the other hand if you are dipped in less water then you should invest with fewer amounts as per your ability. People are now millionaires and billionaires after investing in the Bitcoin and now they are feeling comfortable with any investment.