I'm in the UK and use several NTSC Atari's.
Luckily my old plasma TV and several of my smaller LCDs support NTSC signal on composite and s-video.
For a 400 I can highly recommend SCCC from tf_hh to give fantastic S-video output, this could then be run through an upscaler if you want to invest in that option (retro tink etc but not some cheap ass amazon box)
tf_hh's 48k ram upgrade is also pretty simple to install.
Sophia 2 (grab one whilst you can) will give DVI output which is easy to convert to HDMI with the correct cable, but you will still need to pick up audio from somewhere, so the SCCC comes in handy for that, you will need external speakers or a HDMI audio injector
If you are anywhere near Leicester I'd be happy to show you my set up(s) and help with any upgrades