I would like to get a Nextcloud solution for my cloud/backup needs. I would like to store 5-10 Tb of data. I need to store my passwords there too (so I need an app that gets the passwords from Nextcloud, running on iOS and Windows. I prefer SSDs, and pretty good quality. The cloud part should not require any 3rd party logins. I have fibre internet. Budget preferably below 500 Euros 
I am not a Linux expert (at all).
Thanks!
Nextcloud is not a backup for now. If you synchronise data between clients and Nextcloud, Nextcloud is not a backup, as a synchronisation error can destroy the data both locally and in the cloud.
You can host Nextcloud yourself or have Nextcloud hosted. If you have no experience, I recommend a Managed Nextcloud. The providers differ here. Some providers, such as Shadow Drive, offer you large amounts of data for little money but only the Files and Nextcloud Office apps. Here you may be missing the app for your passwords or other apps. Also you get e.g. only 2 TB for one user. Other providers offer you a complete Managed Nextcloud with administrator rights, where you can install a selection or even all Nextcloud apps. Also mostly you can use your own domain or subdomain with CNAME.
Managed Nextcloud examples - not tested:
Nextcloud at Hosting.de with 5 to 10 TB
Nextcloud with Hetzner Storge Share with 5 to 10 TB
Nextcloud at IONOS with 10 TB
Nextcloud at KeyWeb with 2,5 TB
Here another list: GitHub - nextcloud/providers: community-maintained list of Nextcloud providers
With 6 TB of SSD, you are already around 450 euros. And you have no RAID, no backup disk, no further server hardware.
I read from this that you don’t want to host the Nextcloud at your own home and that the 500 euros refer to a rental per year in the internet.
For 500 euros, it will be difficult to set up a Nextcloud with your requirements at home. As @tflidd wrote you will still need backup etc. If you have no idea about it, then I would advise against setting it up at home. At home, you might be better off using normal hard drives and perhaps an easy-to-configure NAS. But think about backups there too. NAS is also not a backup.
There are also ready-made Nextcloud solutions for home use. But if something goes wrong with updates, it would be good if you had the necessary Linux and Nextcloud knowledge to repair the Nextcloud.