if you don't mind buying games, I'd suggest the series x over the 360, only buy the 360 if you plan to pirate. you can flash the drive if you don't care about dlc. you'll need to look at the jungle flasher pdf for your make model, which you can sometimes get the make by removing the faceplate (at least with phats) and looking at the color of the wires and you can get model by the look of the disc tray when it's sticking out to gauge difficulty without ever opening the system.
can't play homebrew either. what may be easiest is the xkey, but I've never owned one. you can play iso files from an hdd. it's a xode (xbox optical drive emulator), so you need your drive key, which obtaining it will be explained in jungle flasher.
I see you're from Norway, there is a seller on finn.no from Porsgrunn that offers both RGH modding and sells already modified consoles. Don't know about the quality of the soldering though as the person doesn't post photos and I've never bought one myself