Maybe the battery, alternator, regulator, bad ground. Get it started, drive it to a chain auto parts store, and ask them to test it for you. If it is bad and you purchase a battery from them, they will install it for free.

This is in the US, not sure it applies elsewhere.

Some folks change the pads & rotors at the same time. Some folks will have the rotors turned and used on a second set of pads, especially if money is tight.

Personally I fall into the first camp. By doing so, one can drive until the brakes start grinding and then change both. (I primarily do this because I have had rotors warp shortly after being turned but I have never had this issue with new ones.) This does require paying attention to your vehicle, and not always cranking the radio full blast.

It’s all personal preference. Neither way is right or wrong, despite how many people here will tell you differently.

Your first 3 items could be considered consumable, which means that they will need replaced with use. The amount of use & the abuse impact longevity greatly. You can spring for expensive top quality for long life, or you can go for less expensive with a plan to replace more often. If he is working in a super-busy, top-notch shop, you might consider getting the best. Otherwise I would consider Harbor Freight for those items. You can often buy 2-3 of an item so you have spares for about the same price as top quality.

The last 2 items you can get at HF as well. It really depends on how much he is going to use them and how rough he is going to be on his tools.

For an example, the Snap On truck would have a great quality impact gun with a great warranty. But for the cost of that gun, you could get a medium grade impact and adapters, and HF for the first 3 items, for approximately the same cost.

Neither direction is wrong, it really depends on which is more important to you (him)- quality or value.

Your first step might be to go for a car factory to get a tiny glimpse of what goes intro making one. Then think about how you are going to source all those parts. Then how much all of this is going to cost. Then figure how many years it will take to start turning a profit. Then multiply your yearly operational cost by the number of years you don’t make a profit to see how much capital you will need to start with. (Hint- that’s where the $500,000,000 comes from.)

Put me on your customer list, I’m moving to the UK just for this!

If you can hear the motor turning but nothing is happening then it sounds like the regulator has broken. Repairing it requires opening the door and removing the door trim pad. There should be some YT videos to guide you through it.

I know some folks my age that don’t even know what a CarFax is.

I would hate to do this but you might hafta cut the wires, sounds like they may be reversed.

And you will be unlikely to get it balanced, damaging other parts over time.

None of us really know anything at first, we have to learn it first. Well, at first we only know eating, pooping, & crying. We have to learn to eat (chew), talk, walk, etc.

If this interests you, go to school and learn it. By the time you finish, you will have a much better idea whether you want to keep doing it or not. If you decide it isn’t for you, you are not required to stay with it forever.

The lock mechanism and the striker are regulated by the federal government thanks to a safety focused zealot named Ralph Nader. In the 70’s he pushed for legislation to make car doors able to withstand some measurable amount of force as applied in an accident. I do not know if that includes downward force or not, but they are very strong. And when they were a bolt with a bushing astound then they were called the Nader Bolt.

You might start by finding and tightening any ground connection you can find. Google, see if you can find a diagram to locate them.

You are about to learn that electrical problems can be the worst because there are so many possibilities.

Good luck.