03 TXT Upgrade Questions

[TLD]

I'm new to the forum and have been bitten with the cart bug! I have an 03 TXT lifted,22"tires all else stock. Has brand new battteries.Wanting to do a few upgrades. Planning on adding 4ga wires everywhere, Alltrax AXE 4844.Can't spend much right now,but want it usable and dependable. May upgrade motor in a few years. Mainly used for checking cows and to and from the woods hunting, pull a few small hills and plowed fields within a mile of so from the house. Has a wire going into the back side of the box in front of the controller that will get hot after a short ride. What is that part under the box?Will it need to be replaced?.What all needs to be added (on a budget) right now that will get me up and running? All help is welcomed. Travis