Windows XP/7 is probably still the best option, but I don't think they sell any new laptops with windows 7 anymore and windows XP is probably too dangerous because windows stopped supporting it. Windows 8 is quite problematic in general for gaming since it's not meant for that (I bought 1 custom, self-composed gaming PC with window 8. Not a single game worked, so I sent it back.). And windows 10 regularly gets updates that specifically fuck up something with Diablo 2. I know that much from experience. Windows 10 has the other problem of "letter counting" whenever there is something to load (game start up, game creating, video sequences). I'm talking about all the ascii signs appearing in the top left corner when using Glide. It's a strange problem I only ever got with windows 10, so yeah. Win 10 is the cause that Glide doesn't work nearly as well anymore. And the other video modes either lagg or are distorted. At least it's that way on my pc.
The best thing you could possibly do is get any laptop with a SSD as a harddrive (in order to start up the pc faster
). I bought my 2nd-hand laptop with 500 GB SSD for 160€ about 1,5 years ago. Then install windows 7 if you have a copy of that lying around. If not, try ebay for a laptop with windows 7.
But to be honest, hardware doesn't matter. The software is the only crucial thing.
EDIT: just googled for windows 7. There are cheap licenses you can buy online. 10 € or less.