Welcome RocksSilver! I've only been pouring a few months myself, but I will try to help you out where I can.

As far as stamping goes, this can be a matter of personal preference as well as what will fit on your piece and aesthetics. I'm cheap so my stamps are in MM - I tend to use 1.5-3mm on my smaller stuff and 6mm on my larger items. I see a lot of larger stamps being used..  I just haven't gotten there yet. My logo stamps are 6, 9.5, and 12mm and I use the largest one that makes sense on my pour.

If you're using a furnace with a graphite crucible, I personally would not use any borax at all. I have really only seen this used with the hand crucibles. I'm sure others will chime in on this.

As far as buffing/polishing, I'm working indoors due to the extreme cold weather right now and have been using a dremel to clean/polish. I've been told a bench grinder with polishing wheels makes easy work of cleaning up pieces and making them shiny.

As far as temperature goes, I run my furnace anywhere between 2000 and 2025 degrees depending on how cold it is in my basement. You'll probably get a half dozen other answers on this one.

I hope I was able to help you out a little bit!