So it looks like you are an earlier comer than me. Then if so why you need to ask those questions if you are already into bitcoin for years already.
But I will answer some of it.
1. Where I should buy such quantity? Up to now I've been using BTC-E and Localbitcoins to buy/sell BTC
Go for LBC you can see a lot of sellers there and you can compare them and buy the lowest.
2. It's a good time? At today it's 1100$/BTC.. Not the best moment I think, what do you think? Shall I wait?
It's a good time if your a rich guy but if you are not that one, don't buy it's really high.
3. I'm ready to hold it till the next halving (2020 I believe), do you consider it safe (even at the current price)
Let's do it together mate I'm doing it too.
Well, I'm an earlier comer than you but I always played safely. Small investments, more like experiments to test the ground... Now with this decision, I feel the uncertainty to make a new move, and so I had the need to discuss it