Thanks so much for this. This is hepful. Are these expensive? And are they easy to set up?
Retrotink 5x pro is £325. 5x is easy or as hard to use as you want really. You can just plug it in, set your desired output resolution (1080p max, it does have an experimental 1440p, but that's hidden, for advance users), use generic 4:3 or 16:9 (depending on input) and away you go. Or you can really dig in if you like and really start fine tuning it based on systems and add stuff like scanlines, there is a big wiki for it
https://consolemods.org/wiki/AV:RetroTINK-5X_Pro.
Takes Composite, RGB, YPbPr, & S-video.
OSSC is about £130, apparently not quite as user friendly as the 5x (That's what I read when deciding myself, I've never used an ossc). Only takes RGB and YPbPr.
There are other options.
Retrotink 2x pro. This is just a basic line doubler. Outputs at 480p, has a couple of options (smoothing and scanlines). Plug and play, basically no setup. No RGB, only S-video, composite and YPbPr. £130 ish. This is a hard sell at that price, has it's place, but really basic for the price.
GBS-C or GBS-Control (same thing, can be listed as either). This is easily the best value upscaler, but the most faff. It's based on an opensource firmware. Only does RGB & YPbPr in. Good option, has motion adaptive deinterlacing if you ever want to play 480i titles on later systems. Faff comes from they come in different configs. Some only output over vga, so you have to get an vga to hdmi convertor, some have it built in, some don't spit out audio and so on. Can be got for £60ish, but you gotta find the right/good ones.
Lower than that, is pretty much all junk and you may as well just plug directly into the tv.
Upscaling old consoles with no lag and good output is not a cheap game.