i'm planning on upgrading to an 8700k and 1080ti.

which mobo/ram/cooling should i pair with and which model of 1080ti should i get?

usage: playing overwatch at 1080p/240hz while streaming at 720p/60fps.
budget: 2k usd. if you can do it with less, that's fine of course.

purchase date: 1 week

side question: is the 2700x worth considering over the 8700k given that i'll be streaming concurrently? can it fill 240hz?

nn case/hdd/etc. does the extra budget headroom affect your response?

if i have a 550w psu, would i need to upgrade to 650? current psu is evga supernova 550 g2 gold

Just edit out what you don't need. The 550 G2 should be ok, the G2's are gfg power supplies also.

Since the i7 and r7 have pretty similar IPC, the difference in clock speeds and core count would, in theory, make them perform about equal. However, if you were going to OC, I think you could get farther on the new Ryzen chips (J2C got 4.3 stable all cores), plus it's better scaling with higher memory speeds, would ultimately give you a better experience.
That being said, I don't think you will be able to maintain a steady 240fps @ 1080p max settings while streaming 720p60 with either chip TBH.

Video for reference https://youtu.be/ynFQSfIE9H4