Do they allow installing any Nextcloud app of your choice? I could not find that information.
Yep it works! Obviously if the app requires specific software installed on the server it may or may not have what you need, but I've installed several apps including the Music app, and use mine as a Subsonic server.
May I ask for three specific apps? Contacts, DAVx5, and Memories.
that sounds like an awesome deal, but the latency for me in the US would be painfull. Looks like they do not offer US hosting for this.
They don't. FWIW it's never been an issue for me in the Midwest, but YMMV. In general I'm syncing the files I need to my computer or backing up photos to my phone so latency doesn't really matter for my use case.