

Keep browsing eBay and HotUKDeals. You can easily find something really good for <£200.
This 11th Gen Intel Dell laptop was going to £150 this morning and I was on the verge of ordering it and adding more RAM.
This sub is obsessed with Thinkpads, but when you’re looking for a secondhand bargain then you pick up what comes opportunistically. The market is flooded with Dells because businesses change machines long before they go out of date and generally keep them in very very good condition.
I’ve just finished setting up exactpy what you describe (Synology NAS and Debian mini PC server) and it is working so well. Why do you need a more powerful NAS? If you use it for just storage you won’t really be falling short of NAS compute power.
I considered adding the drives directly to the mini PC as well, but I’m happy I’ve kept the Synology for the best of both worlds. Synology makes it so much easier to share the storage, set up reverse proxy, provide constant 2 way sync with my online cloud WebDAV service, etc.