Few of us want a server-side windows box 
The first thing you want is an SSD, preferably NVMe, to host the database. The catch with Roon as compared to LMS is that you want really high IO, both throughput and iops, on the server side. Past a certain threshold, I’m unsure how much the Roon experience can be affected by the fastest SSD in class, but the Samsung Pro NVMe line seem to be a common recommendation.
No matter what you do, please don’t try to host the database on a spinning drive. The reason to this is that because the tracks are interconnected, you’ve got a very quick (not quite exponential, but still steep) rise in load the more tracks (local or remote) you add, and your 2,5" rust bucket will choke, limp, and make your experience truly miserable. I haven’t checked this with RoonLabs, but I also suspect that there’s a difference whether or not you’re subscribed to a streaming service (it’d make sense that a library of 10’000 local tracks shouldn’t interconnect as much as 30’000’000). There might also be a difference depending on the type of albums you own: 10’000 tracks of best of the decade compilations might carry a much higher metadata load than 10’000 tracks of New Kids On the Block bootlegs.
Regarding your laptop, the official guidelines in terms of processing requirements are i3-7100 up to 100’000 tracks or so, i7-7567 beyond that. They are generous, and you will get away with (much) less, but that’ll give you a ballpark of what’s expected for the best possible experience. I’m currently running about 100’000 tracks on a Xeon D-1518 docker container, and it’s perfectly useable, but a touch laggy.
As far as tryout is concerned, as long as you’ve got an SSD on that old laptop, try that and see if it works. If it doesn’t, temporarily run it off the most powerful box you have, then migrate to something more economical. ROCK (“Roon Optimised Core Kit”, or RoonOS) is supported on NUCs, but runs fine on other setups (those are called “MOCK”), albeit with no guarantee something won’t break down the line: it’s a lean and mean, closed distro meant to be as appliance-y as possible. Since you’re comfortable with DiY’ing, if you really want to cheap out, you can probably scavenge a known compatible motherboard from ebay or your local classifieds. As far as NUCs are concerned, the sweet spots right now, as far as I can tell, are “really cheap 2nd hand if you can find one”, or “NUC8”. I’m personally unconvinced the limited financial gains by going the MOCK route are worth the (small) risk of something breaking down the line, and feel like outside of edge cases (million track libraries…), the best bet for those of us who know how to build a dedicated box is to use the continued support safety net from Nucleus sales and get a NUC.
Regarding your Raspberries, the best-in-class endpoint Raspberry distro is called Ropieee, so you can do that if you’d like a more appliance-like experience than DietPi.