The WiiM Pro is a streamer. It looks like you found some listings on Reverb and eBay with good pricing. The only thing is if you buy used or off eBay and get a defective unit, it might be hard to return and get a replacement.
If you were to get a WiiM and you want to use the app on your phone to control playback, you’ll need to enable ’Media Streaming’ on the Windows PC that stores the music files. This will turn on the UPnP server. The WiiM app should then be able to locate your Music library on your PC. There are plenty of how-tos if you do a search on Google.
Otherwise, you can install Stream What You Hear (SWYH) on your laptop and stream all the audio on your laptop to the WiiM or the Onkyo (if you connect it to your network). SWYH is a UPnP streaming app for Windows. Have a look at the ’Getting Started’ guide - link.
If you want to experiment without having to buy anything, try connecting your Onkyo receiver to your network. Here are the instructions for setting up the wi-fi connection - link. If the receiver is close to your router, then try a wired Ethernet connection instead. Download and install SWYH and follow the steps in the Getting Started guide. See if you can successfully find the Onkyo as a UPnP renderer and stream to it. I’ve done this before with my PC and a Raspberry Pi as well as an Oppo UDP-203. It was pretty straightforward to get everything working.
