Hi there, glad you are enjoying your printer! I got mine a few weeks ago, but have been printing for some time on other printers.

The LED on the build plate indicates when it is heating. You no doubt have also seen the LEDs on the limit switches as well.

Can't comment too much on the build plate. Mine is flat to within 0.05mm, so good enough. Glass would be your best option, but clip it, not glue it, as otherwise it will just follow the dip in the plate underneath. Mini bulldog clips - a couple on each end will be enough to hold it into place and don't take up too much build space.

Most of the nozzles you see advertised will fit, as it is a standard thread and it's not too fussy about thread length. You may need to tweak the height by taking off the fan on the front of the extruder and nudging the tube up and down. Stainless steel nozzles last longer and allow you to use a wider range of filaments. Worth the extra money.