Start small. Find good used hardware first before thinking what services to run. I would start with an old desktop.

Self-hosting is a journey, not a destination. No matter what you buy you'll probably need to buy new hard drives. Used hard drives are a bit of a gamble.

Where do people buy used systems in Denmark? Show us a few things you're interested in and people can give you recommendations.

Also, instead of Photoprism, I would suggest Immich. I was a huge supported of Photoprism for years (even donated money) but their development is too slow. Immich is way faster and has an android app. Anyways, give it a look.

I think 8 GB of RAM is sufficient for all those services. I run them all with Yunohost and I rarely get over 4 GB RAM used.

I use this, too! It's great but doesn't always work.