Kind of both. You get to decide who you federate with but deciding who ferdarates with you is opt out. By default your server doesn't know any other server. But you can search for communities on other instances by taking the url of the community on the other instance and entering it into the search field. This way the other server gets indexed by your server and after that federation is automatic
Your best bet is probably to get a refurbished thinkpad. I've seen X1 Carbons (their ultra thin Macbook-like line of ultrabooks) for 250 bucks on backmarket. That's the 2016 model (G5) and not in the best tier of refurbishment (So there might be some small scratches on the chassis) but the hardware should be more than enough for your usecase. Linux generally speaking does very well on low end hardware and thinkpads are especially well supported.
One thing though: DON'T INSTALL KALI ON YOUR DEVICE, IT'S NOT DESIGNED FOR THAT. Or rather it wasn't. Originally Kali was meant to be run of a thumb drive only. By now enough people have attempted to install it that the devs have given up and provide a bare metal edition. That is just Debian with some stuff preinstalled though. There is very little secret source (like the "undercover mode"). You should go with debian and try to build your toolbox as you go. This way you will get a better insight into your Linux system because it forces you to set up everything yourself
E-Mail is a federated standard, so in that sense every mail provider is part of the fediverse.
I think you are confusing fediverse with non profit and/or open source
If that is your question than yes there are alternatives out there. For example murena, the organisation behind the degoogled android fork /e (more information)
Here is an example of how to use this:
The code  produces:
(I've taken a random picture from Unsplash for this. It doesn't mean anything more
One way to lighten the load on servers is to host your images externally. You can still embed the images into your post and nobody will be able to see the difference on first glance. The markdown to embed is: 
To be fair: Most of the P&C designers left the studio years ago, so I Deadalic closing its studio isn't that much of a loss.
Bei einer DDOS-Attake (distributed denial of service) wird ein Server mit mehr Anfragen überhäuft als er verarbeiten kann. Als Ergebnis kann niemand auf den Server mehr zugreifen, weil die Leitung "verstopft" ist.
Twitter betreibt ihren Dienst nicht nur auf einem Server, sondern auf vielen gleichzeitig, die alle miteinander reden. Die Webseite ist auf einem Server, die Datenbank, in der die ganzen Tweets gespeichert sind auf einem anderen.
Die Webseite scheint so programmiert zu sein, dass sie weiterhin den Datenbank-Server nach neuen Tweets fragt, auch nachdem die 600 Tweet-Grenze erreicht ist. Die Datenbank liefer die Info "Limit erreicht" zurück, aber keine Tweets, also fragt die Webseite weiter nach neuen Tweets. Etwa 10 Anfragen pro Sekunde. Dadurch hat der Datenbank-Server mit mehr Anfragen zu kämpfen als er verarbeiten kann und liefert auch an Nutzer, die ihre Grenze noch nicht erreicht haben keine Tweets aus
Scheint so, als würde das gerade passieren
Siehe Stündlicher Zuwachs in Hellblau. In der letzten Stunde gab es 3225 Neuanmeldungen nachdem sich über die letzte Woche die Anmeldungen bei knapp 500 pro Stunde eingepegelt hatten
Und anscheinend DDOSed sich Twitter gerade selber durch diese Aktion
https://sfba.social/@sysop408/110639435788921057
Twitter is firing off about 10 requests a second to itself to try and fetch content that never arrives because Elon's latest genius innovation is to block people from being able to read Twitter without logging in. This likely created some hellish conditions that the engineers never envisioned and so we get this comedy of errors resulting in the most epic of self-owns, the self-DDOS.
I don't know what to tell you. Makes sense to me