I have read several posts on this forum where people have claimed to have invested all the money they have on Bitcoin, I also read a post where a guy claimed to have used all the $20,000 his uncle gave him to also invest in Bitcoin, some even sold their landed property. The reason for these risks and others is the firm believe that Bitcoin would be $50,000 - $100,000 by the year 2019 or thereabout.

First, I must say that I admire the courage and high risk appetite of such persons. But, I must also warn.
Yes, It is good for everyone to have high hopes for Bitcoin, and also hope it gets to 100k USD in future, but you should also consider a safety factor in anything you do including investing.

Mathematically,

DO NOT Assume,
P(Success of future term) = 1 ; and
P(Failure of future term) = 0

This implies that, you must always leave a little fraction for probability of failure which in this case is the safety factor, no matter how little the fraction may be. Simply put, you don't have absolute control of what happens to Bitcoin and Cryptocurrencies in the future.

Invest wisely, only invest what you can afford to loose

Just a piece of advice.

What are your own suggestions?

Very good advice.
In fact, I think that everybody should understand that crypto market is the same as any other market, stock market, Forex etc.
It's very risky so please be careful before investing.
The best strategy, in my opinion, is to invest only money youcan afford to loose and to diversify your investment.

I don't believe that investing all your money in crypto is more dangerous than investing in anything else. THere's always a chance of failure in everything you do. You can fail by getting cancer and dying, being killed in your car while driving home, getting robbed. To me cryptocurrency offers a very high probability of success. Much greater than real estate and other so called safe investments. When you're poor you have to take risk or you will never achieve anything.

I agre with this quote because I definitely believe that in everything you must have a fallback. It's not that I meant that you're gonna fall but what if you change your mind? At least there's something left that you can use in other way to earn profit. Above all else bitcoin is still developing so you can try another way of earning.