Link Building during Sandbox

As a business owner I'm absolutely new to SEO and as for now I try to figure out the effectiveness of SEO company (which promotes my websites).
I'm interested in the following questions:
Do you believe that sandbox still exist?
Is it a good idea to buy several links on brand new domain and forgot about it on 3-4 months?

In my niche it is a big competition and I'm trying to figure out how to start new websites.

Thanks!

I'd like to know the answer to this as well.

Are there any professional webmasters that can say how do they feel?
Thanks in advance.

Doesn't matter whether the sandbox exists or not. If you're going whitehat, start slow, and build links at a steady velocity. Eventually you'll rank. SEO takes patience more than anything.

Thank you!
For how long do you build links slowly?
Is it 3 or 4 months?