Hi, and welcome to the club!
The monitor may be a 'hit and miss', although some of them handles well the non-VGA standard resolution and refresh rates (i.e. the 720x576@50Hz PAL standard) as well. If no, you may still solder/purchase a 'minimig VGA to SCART cable', it works in most cases. About aspect ratio: native 4:3 is the best, I think on monitor-TVs there should be a possibility to set it manually. I recommend to use a CRT with SCART input, it gives a genuine retro feeling to the system, and they are cheap, if you are lucky enought, maybe you can get one for free here in Europe. I suggest to wait until your MIST arrives, and test it with your Samsung.
1A USB phone charger is more than enough, i use a Huawei branded, it's flawless.
Standard USB mouse and keyboard is OK, I have no experience using trackballs.
Atari joysticks: they are still manufactured, i tried two of them:
Competition Pro: precise microswitches, durable mechanism, but no autofire, and a bit too stiff for my taste. Costs 30€+shipping.
AFF Kolor: no microswithes, but still seems to be reliable. Has autofire, and costs less (20€+shipping). Short (only circa 1m) cord.
I purchased them from Amigastore, they are correct, but a bit slow.