The most important bit is the soldering iron and the connectors you plan to use.
Since you will be re-wireing in most cases it's an opportune time to move to deans connectors. To efficiently solder deans you need a soldering gun rather than an iron. And some way to hold the connectors as they get hot. A decent flux, and decent solder are also a bonus.
Machine mart sell a kit that isn't as cheap as some of the Ebay deals, but it does have everything in it for deans.
https://www.machinemart.co.uk/p/csk100-universal-soldering-kit/
The holder with the crocodile clips will hold the deans and wires, the soldering gun will give you the heat, there is also a pot of reasonable rosin flux and some solder.
I would add 63/37 eutectic solder to the shopping list as well. It's a specific alloy that goes from liquid to solid without the mushy phase of 60/40 and lead free solders. It also melts at a decent low temp, Lead free needs more heat, and will make you cook the connectors.
You will need deans connectors, and a good supply of heatshrink tubing. Ebay is the best place for heatshrink, you can buy a box of multiple sizes and colours cheap enough. And having the right heatshrink will improve the life of any loom you make. And most importantly it will give you the best protection against shorts and look the most professional when done.
https://www.ebay.co.uk/itm/400X-Heat-Shrink-Tubing-Tube-Sleeve-Kit-Car-Assorted-Electrical-Cable-Wire-Wrap-/273157170719?_trksid=p2349526.m4383.l4275.c10
The wire is your choice, but I've had good results from the silver plated type. AK2M4 sell this cheap compared to most shops that are double the price.
https://www.ak2m4.co.uk/internal-parts/misc-internal-parts
For the actual units themselves you have a lot of options. AK2M4 again off some of the cheapest I have seen if all you want is basic protection.
https://www.ak2m4.co.uk/internal-parts/mosfets
The Xcoretech x304u is also a cheap well respected unit for basic protection that follows the same design idea as AK2M4's. I have one of these in one of my augs and it fits without a problem in the space behind the gearbox plate.
If you want active braking or circuit protection then Gate make some very good fets. Expect to pay more for them, but models like the Gate Nano hard offer on/off active braking, and user definable circuit protection. Again I have one of these in an AUG but it required significantly more work to get in, And I had to cut the gearbox plate.
V3 gearboxes can be fetted without opening them. It's as easy as cutting 2 wires and soldering some longer leads in place to the fet.
V2 gearboxes have to be opened. You need to route in the signal wires and change the motor wires, so V2's are significantly more time consuming, and it is worth totally replacing the loom in most cases.
