I would love to build a retro console and would like to know the cheapest way to do this. I would love the original controllers to work with each emulator and for it to work with hdmi as getting an old TV is not possible (it's the only thing that's holding me back from getting the original consoles)
Is a raspberry pi and getting usb controllers the easiest way to do this. Also if so can the what can the raspberry pi comfortably emulate, dreamcast, n64.
Obviously if there is another option that isn't a pi let me know
Well, I mean, if you have an old laptop lying around, you could just use that instead
But, AFAIK, the Raspberry Pi 3 can emulate everything up to the PlayStation rather well. You may need to overclock your Pi's CPU in order to get the newer systems to run at full speed. N64 and Dreamcast games should work well, but there may be compatibility issues here and there. You'll want to stick with RetroPie, which organizes emulators for a bunch of different systems in a nice and organized fashion. It's available not just for the Raspberry Pi, but for Windows, Mac, and Linux as well IIRC.
And yes, the Raspberry Pi supports HDMI, so again, if you don't have an old laptop lying around, the Pi 3 is your best option.
As for getting the original controllers to work with each emulator... You're going to have to configure each controller separately in EmulationStation, the front-end launcher for RetroPie. I found a set of USB Adapters for a few systems here:
NES Adapter
Mega Drive/Genesis Adapter
SNES Adapter
Unless you're planning on emulating
every single system, you might just want to stick with 2 or 3 systems for these controllers and call it a day, or else you'll have controller ports lying all over your system.
Also, and you probably don't want to do this if this a beginning project, but you could perhaps wire your emulation system to read original cartridges like what this guy did here:
https://hackaday.com/2009/06/19/usb-reader-for-snes-game-carts/
You might also want to get a case for your Raspberry Pi. You can purchase one on Amazon, or 3D Print one yourself. There are a lot of 3D Cases that are made to look like old gaming systems, so search around, and see what you like.
That should be about it. If you want to find out more information, try searching the tubes, or look up "raspberry pi retro game system". You'll find a lot of useful results.