First, figure out what you have on both ends - I suspect you have a 2" receiver on the car, and a 2" ball is needed for the trailer's coupler.
The piece that you mount the ball on that connects to the receiver is called a draw-bar or a shank.
It does not matter if the parts are rated for 3500, 5000 or 6000 lbs. They will all fit the same. Mostly what you'll find in the stores are 5000 lbs. All that is important is that they are rated at the weight
limits you are dealing with or higher.
If you have the smaller reciever on the car, it will be 1 1/4" square. All you'll need then is a draw-bar of that size. Generally, these max out at 3500#.
As for your wiring, 7 pin is the way to go, but that may not be what you have on either end. Many small cars and older trailers only have 4 pin wiring. So you may need some converters on hand to make it all fit up. But bear in mind, electric brakes
only work on a 7 pin connector.
One last thing: It's very important that your trailer sits level when you are towing it. Draw bars come in a variety of types to make that happen - some have drops, and some have rise, and some come straight out. You will need to know how high off the pavement your trailer ball must be to have the trailer sit level under tow. In the case of my Oliver
, it was 22" and needed a straight draw bar for the ball to be at that height. Most small trailers are 16", but they do vary. Plus or minus an inch is OK, but no more than that.