It's a rabbit hole, and the cost can easily pile up, not even talking about the time spent studying all the possible mods and brands and styles, but at the end the satisfaction is there and I think that my custom keyboard is built to last a decade or more.
Using it for typing code, prose or instant messaging is a delight. The type of experience is similar to what you get when driving a luxury car or wearing tailor made clothes, it feels good.
The base board I used is the GMMK pro, not the best choice if I had to do it again, I had to heavily mod it to achieve the sound profile I wanted and it is still a bit stiff.
I'd recommend buying something a bit pricier, built from small companies in the US or South Korea, those are in my opinion the most advanced design at the moment.
I used lubed Gateron Black Silent for the switches, very happy with the result, smooth, quiet with a bassy sound.
MT3 is in my opinion a very good choice for the keycaps profile, sculpted and retro, I used the Susuwatari from Drop.