This could be a big can of worms!!
If you can afford a Komet Butterfly, get one! I trained in a Buckingham master II ('cause it was the only small one they had) and bought a Butterfly when I started on my own and I love it!
It's light to start with, is incredibly manouverable, has a wide comfortable back support. It has three nice big tool loops on the back, the middle one is great for the saw, keeps the weight central, two big D rings on the side, and a long bridge across the front so your attachment crab can slide side to side. Some people are leery of the quick release buckles, I inspect them before and after each job and keep silicone spray on them, the buckles are just a positive as the day I bought it (2 years) Remember, a tree harness supports your weight, It needs to be comfortable to be in for hours!
As for boots, I use my chainsaw boots when climbing, spikes or not, they have nice wide stiff soles that keep my feet from getting pinched and are good for kicking dead stubs off with!!
Good luck with your training, enjoy