You can't build a laptop.
I can build a laptop, but I've got thirty years experience.
Repairing laptops is easier than building them, but not much cheaper than just buying one. The problem is, most parts are all integrated, and seemingly simple fixes often require replacing an entire board. It's only really worth it if you can buy a job lot of several of the same model of laptop. If you get an ebay ad of "6 exactly the same laptops, spares or repair", you can normally get two or three working laptops out of it. If you buy one, and try to buy parts, it's normally more hassle than it's worth.
If you're building your own, do yourself a favour and at least start with a desktop.