If you want to start try to find something used on DBA, like an old laptop.
If you are an student, maybe someone in your class upgrades their laptop and you can get it cheap.
(Best a laptop where you can remove the battery, plus you need to change a setting so it doesn't go in standby when closing the lit)
You can add an external hard disk for nextcloud data.
My first home server was an raspberry pi, it's not great for nextcloud, you need to disable all preview image, and the UI might still be slow.
Untop using an microSD card for the OS might randomly break (happens to me, SanDisk).
My second server was my old laptop, I used an laptop with i3 from like 2013 as server for a long long time.
Best thing I can recommend is to not rush and get the first best thing, try to look for a good deal.
Start small and you can always increase your server in the future.