If it were me, I’d spend £60-£70 on a Unifi AC-Lite or similar and run a cable from the router to the AP upstairs, this will give you wifi coverage upstairs for minimal cost and can easily be expanded. Yes, you do need to plug in a PoE injector somewhere, but going this route is way under budget and only requires minimal effort and keeps your other devices wired for maximum speed and reliability.
Plex wise, if you are having to transcode local content with Plex, you are probably doing something wrong.
Look at the three C’s:
- Content - If you make the wrong choices when encoding or selecting the content on your server, you needlessly have to transcode. If your device doesn’t support h265, then don’t download h265 files.
- Connectivity - High bit-rate and wifi are not ideal, so don’t do it. Also multiple clients on wifi or your neighbours making poor choices will exacerbate the situation.
- Client - Pick clients that can direct play your content.
So shoving an underpowered 1st gen. FTV stick between a solid wall and an RF shielded screen and expecting it to stream 50Mbit 4K HEVC via wifi is probably not going to end well. Hard wire a 2nd gen FTV4K or better yet a Nvidia Shield in and you’ll not have an issue. Either way local transcoding shouldn’t normally be a thing, except in specific or exceptional circumstances. Eg your lounge set-up supports exotic audio formats and your bedroom doesn’t, or for example XB1/PS4 require a remux to direct stream, obviously remote clients are a different story. You mention not being able to transcode 4K, honestly, you shouldn’t be, especially not locally. Transcoding 4K H264 is resource heavy, circa 14-16K of CPU Mark depending on the bit-rate, if that 4K happens to be H265, it’s way more intensive, and if it’s HDR, it’ll look like washed out crap anyway. Also 4K transcoded is always output as 1080/8/SDR by Plex (at best), so you’d be better having a 1080 to begin with. This is why you normally use something like Tautulli scripting to prevent people transcoding 4K content and ideally split the library. Things are slowly changing with HDR, but the upstream fix for tome mapping is still not going to be penalty free when it becomes available and usable.
If you did go for the google wifi option, then no, the PCIe bus would be fine, wifi is relatively low in terms of bandwidth used, USB 3 and an extension lead would likely be just as quick and allow you to move the adapter to get the best possible signal quality though.