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.
dba.dk is a pretty popular site for buying used stuff in Denmark, but for electronics I usually go on eBay and sort by EU only (IIRC they removed that option so now the results are tainted with lots of UK gear that'll be hit with import taxes).
Both deals sound amazing to me, but get 8GB or prepare for RAM upgrade. 4GB could be enough for what you listed there, but you might find more services to run in the near future 🤪
I think those tiny PCs are perfect if you dont need more SATA ports. Its hard to beat them with that low price