Question: I want to build my own laptop from scratch. How would you go about it, or what would you advise me to do? (and please don't say: buy a new/refurbished one)

* Is it handy to combine elements of several brands?
* Should I stick to just one brand or several?
* is it possible to find a generic casing?

Thanks.

Answer: 
I would suggest looking into 'barebones' laptops. They include the bare minimum of parts (usually at least a motherboard and CPU), allowing you to do the rest yourself. There are a bunch of companies that sell them in varying degrees of completion, but the only one I know off the top of my head is Clevo. They are what a lot of the companies who sell customized laptops, such as Sager, use as a base.

As far as brands go, it depends what you mean. Some CPUs play better with certain motherboards than others, and I think the same goes for video cards (sorry, it's been a long time since I've bought one). Your RAM sticks should definitely be the same, ideally identical. For hard drives, the brand shouldn't matter as far as compatibility goes. There are only a few major brands and I've never had any issues mixing and matching.
The main parts to worry about compatibility with are the motherboard, CPU, and RAM. A quick Google search can usually tell you what a certain model will work with.
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> Is it handy to combine elements of several brands?
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Apart from a few parts (RAM, SSD, CPU..), **no**, parts like keyboard, motherboard, battery... are not interchangeable, often exists several standards even within brands

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> Should I stick to just one brand or several?
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As mentioned above, even within one brand parts might not be compatible.

> 
> is it possible to find a generic casing?
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I'm not aware of that.