Do you consider a nice laptop an investment? (I'm planning to buy one and pay it with credit for 15 months)

Depends on when / where you need a computer and what you need it for

Mostly development.
I've got a thinkpad t420 with 8gb ram at my job but sometimes I'm in the need to run visual studio/eclipse/chrome/vs code all at the same time and while the laptop won't freeze, it gets annoyingly slow. My boss is pretty flexible since we use tfs to work on projects and I'm sure he'll allow me to use my own pc. Furthermore, I want to star writing games with unreal or unity.

Unless you need to be able to bring it with you, get a desktop instead. more power for less money

What @svgPhoenix said. A desktop for horsepower and comfort at home, and a portable - above all - laptop for outside,if needed.

Anything with an ssd and 16GB of ram will work.
If you have a restricted budget ssd + 8GB ram (you will use swap)
Check the monitor it MUST be a 1080p at least.

Look at lenovo,dell,hp

Sounds like you're buying a laptop for work. why doesn't your employer provide you with a suitable laptop?

I suggest you to not spend your own money for something you are gonna mainly use for your employer. Just save those money if you can and maybe buy a desktop (cheaper and powerful) for yourself.
Your boss is not making you a favour letting you use your own laptop, I bet he's more than happy...

As a freelancer: yes.
As an employee: no. As an employee I expect to be given a decent device that I also can use in private.