It's really not hard, these days it's a lot like building a computer. You can find the individual components you need pretty easily. There are four things that are essential to an ebike and a whole bunch of quality of life things you can get. All you really need is the motor, the drive computer, the battery and the frame.
If you're getting into ebikes as a total beginner with no idea where to start I'd say the first decision you'll want to make is whether you want to go with a mid-drive motor or a hub motor (9/10 times you'll want mid drive). Do a shit ton of research on the pros and cons of each and then decide which one fits your use case best.
Then, I'd get a conversion kit in whichever drive type you'd prefer. If you go mid-drive I can solidly recommend the Bafang conversion kits. There's a ton of components that will come with them, so don't worry about the budget really, just expect to spend approximately $1,000 and then do research on the size of battery you want, and what features you want out of the drive computer, then select them when you purchase the kit.
It's hard to select some baseline components that I can generally recommend because honestly it's hard to fuck up the part selection, it will probably work no matter what you pick, but I'd recommend really researching each component to make sure it's what you specifically want.
Finally you'll need the frame, and it helps to have experience fucking with bicycles, but it's not required. You'll also need to either buy some tools or enlist the help of your local bike shop techs. There are a billion great videos on youtube on how to assemble the Bafang kits, they're great, you should really stick with Bafang ngl.
Also, and this is coming from someone who's built two ebikes and is trying to resist the temptation of building a third; don't build an ebike.
It won't save you money, they're not more reliable, it's going to take you a couple weeks to assemble and the guy at the bike shop is going to tell you it's a death trap to convert a 1990s steel frame mountain bike into an ebike and they're right.