One gonna need at least a few grands worth of strange equipment normally used by people who repair electronics (the strange equipment used by manufacturers costs millions). Having prior experience with these tools is recommended.
Another obstacle is 3d printed case, phones need both high precision and high strength, AFAIK many 3d printers/materials lack at least one of these properties.
Still it looks doable, and apparently some people actually did similar stuff.
You can certainly do microsoldering in $1000 area now. Not sure about BGA as I have not done those, but I can imagine an oven wouldn't cost more than $1000 as well. There are so many affordable tools available now, it is within reach to build your own phone.
Phones are small, you gonna need a microscope or at least a magnifier with good lighting. BGA is not too hard but you need these laser-cut reballing stencils, ideally not universal but specific to particular chips. AFAIK lead-free solder is very hard to do at home, to use leaded solder you gonna need a setup with good ventilation.
If the goal is to make just a single phone, and you don’t already own all that stuff, I’m afraid the required equipment is going to be above $1000. Maybe if you live in China will be cheaper, but not for the rest of the world.
Another obstacle is 3d printed case, phones need both high precision and high strength, AFAIK many 3d printers/materials lack at least one of these properties.
Still it looks doable, and apparently some people actually did similar stuff.
http://alumni.media.mit.edu/~mellis/cellphone/index.html
https://diy3dprinting.blogspot.com/2013/09/diy-mobile-phone-...