Hi if you have the single pivot calipers they are not great. I recommend upgrading the innercable to Teflon lined. I don't when brompton started fitting them as standard. Also, I went for Teflon lined outcable. Koolstops work very well but so do other makes of premium pads. Upgrading to current brompton twin pivot cost about 90 pounds. You can modify standard brakes but it's a pain to find the parts to reverse the pull direction.
Bottom braket is square tapper, you can swop the crank over no problem as far as I know. You will need a Steven perry or simular bracket if you want a front mech, else go for a dirty change(manual shift).

Many SA hubs models about. Make sure before getting internals. Its not difficult job to oil or replace the internals of a IHG. I am clueless about their black magic, but I managed to swop over the internals of a dual drive hub easily enough.

Your rims are strong enough but jur recomemds using just the front brake for routein braking to extend rim life.