In donorbox we can set up a Monthly or Annual Membership:

I need to know what will be the values for the tiers and if we will have both membership types.

We already have a form up for the membership campaign that I had created last year,
https://donorbox.org/kde-community-membership

After the tiers are filled, we can embed the form in our pages.

So I did a lot of digging (which is why this took so long).
Here is where we are: in 2014 we voted to have flexible membership fees. The wording is "Amount selected by the supporting member, at least 20 EUR per year". It seems this is by now contrary to what the board wants (flexible but at least 100 Euro a year or 30 Euro a quarter) but we voted on it.
That leaves 2 questions:

1. Is it possible in donorbox to set a recommended donation amount, a minimum amount and a custom amount? The idea would be that we'd ask people to give 100 a year as recommended, 20 Euro as minimum or whatever else over 20 Euro they want to give.
2. Do we go with what we voted on or do we reopen the vote? I'll bring that to the board call on Thursday.

1. No. Custom amounts can only be set on common Campaigns, like the one that we have here: https://donorbox.org/kde-community
2. I would like to have the board opinion on this. We may check the data from our ongoing campaign and check the average donation and start from it(lays check the gross data) -> The average donation is 30 Euro per donation. 10 euros is the average for the monthly donations and 60 euros for annual donations.

Note: Membership on Donorbox can only be set for month and annual donations, we can't configure for quarterly donations.
Based on this article from Donorbox, we can use a common campaign for a membership where we have the flexibility for it: https://donorbox.org/nonprofit-blog/recurring-donations-memberships
Apparently, there's a difference between membership and regular donations on the tax side that would be worth taking a look.

In other words:
We can move the membership users of CiviCRM to the ongoing campaign that we already have and add the quarterly donations option, and change the amounts (10, 50, 100). Or we create a new campaign for membership and remove the option of monthly and annual of the current ongoing campaign, allowing only for one time donation.

Decision: we'll move forward with 25 Euro per quarter or 100 per year for now.

@lydia the website is up: https://donorbox.org/kde-become-a-member

Question: Should anonymous donation be active for membership?

Depends on what anonymous means there. We (as in the board) need to know the name of all members.