/e/ is Android, too.
Android is a base on which smartphone vendors and projects like /e/ (there are more) can build their OSes. They might turn out to be different in certain aspects, but they all use the Android base, and for this they are all Android.
Yes.
You can check availability of Apps here … https://e.foundation/apps.html.
Mind the note there, you need the “codename” of the App, e.g. com.spotify.music or com.reddit.frontpage (you can get those names e.g. from the URL in your browser address bar when you open the Play Store page of an App).
Apps currently not listed in /e/'s Apps installer can be requested by you directly in /e/'s Apps installer. /e/ developers will then see to it that the requested Apps get listed if possible.
Be aware that /e/ does not contain the Google Apps and services which come pre-installed with Fairphone OS.
That’s the point of /e/, and you are probably interested in /e/ because of this, but there are many Apps out there which are built in a way that requires those Google Apps and services to be present.
Many of those Apps work flawlessly nonetheless on /e/ thanks to the included Open Source replacement called microG which tries to mimic some crucial Google services, but there will be no guaranteed compatibility.
If it comes pre-installed for you it will basically be the first-time Android setup routine you already know … but missing some Google stuff this time.
/e/'s Apps installer works as any other App store, so that should not be a problem.
/e/ is officially still in beta state currently, a version 1 is supposedly on the way.
That being said, a lot of people already use it as their daily driver OS.
In case you don’t like it, you can go back to Fairphone OS quite easily … https://support.fairphone.com/hc/en-us/articles/360043337512-Install-Fairphone-OS-on-Fairphone-3-using-a-computer
As for bugs … Fairphone OS has bugs, /e/ has bugs, every OS has bugs, just look around in the forum whether something catches your eye and try to get an idea of whether it might be singular cases or whether it’s general bugs affecting everybody … https://community.e.foundation/search?q=tags%3Afairphone
To give you an idea, here are the updates so far for the users who initially installed /e/ themselves … https://images.ecloud.global/dev/FP3/
Pre-installed phones currently get those releases a bit later after they got tested a bit on the none-pre-installed phones.
Of those updates, one was a “hotfix” (the 20200611 one right after the 20200610 one), so bugs might get fixed pretty fast if they are being considered bad enough by /e/ staff 
. Apart from that /e/ seem to aim for at least matching Google’s monthly security update schedule.
