Personally: I usually focus on the Monitor and keyboard. A bad monitor makes for a poor Terminal no matter how fast the cpu. Good monitors have very good contrast ratio and 72Hz or better. Good keyboard I think are no longer made (all are dang silicon mushroom type)
On the other hand if you plan on "building Android via google's instructions", you may need 128GB memory not including disk space: and that's gonna eat into the budget.
Another consideration is if you should get "a really kick as cutting edge Smart Android TV" (for your money) and hack it to be a linux terminal (without disabling the TV - ie install Termux) with an SDD card for extra space. It's a new wild world I can't give you advice if you'd like it: it would NOT compile android - you need a PC for that, currently.
(i want to justify that since it's new info: after installing Android on an android device you can run "some linux", and they are currently improving how much. you can compile but not run the gambit of re-compiling Android using Android YET. they are working on it presently)
(you get android gaming with an Android TV btw - not cutting edge but depends on what games you like: ie, rpg may work really well on Android TV)
here in the US, i built a PC (i5, msi, 8G/1TB) for about $500 in 2016
including logitech PS2 controller 
the MB was free with purchase of CPU full price, the video card was on the "returned" shelf. The monitor was separate. years later i am using an android smart tv as my computer monitor, actually
In my opinion it's a question of future income: you have $1200 now. But when can you afford to upgrade next? I'd say go for I9 if you can afford to upgrade your monitor "in a quiet amount of time", otherwise suggest you hold back and get a monitor "you will be peased to look at".
HP all in one. Touchscreen. They aren't "gaming" but can be pleasant. They are a complete tight package: everything "just works".
Imac. Again not great at gaming (well - depends on model): but has unix-like OS (not mint compatible), and a pleasure. Has games on Apple Store.
NOW. For linux. Kali may not stay up to date with REDHAT or UBUNTU/Debian.
You REALLY REALLY need to get "double verified information" from Kali users what HW currently works on the current version of Debian/Kali.
ALSO: they update and break things frequently (and even target certain brands "they don't like - so i say"). It's often been the case someone with a "worknig setup" suddenly looses all gaming period even vesa X logins too (they are told "maybe works next update" and left to hang out to dry). Abandonment warez, abandonment philosophy.
Me? Graphics on the new i9 (the one that has graphics on it) is so good I would game without a buying a video card. I like RPG games though and gta5 rarely: i'm not very demanding. I would get i9 and no video card to start. but it would NOT work in linux i'd have to run win10 / Steam for games
my take on Steam/Ubuntu is: it's 32bit required. there are extremely few RPG they are all 2x the price as win10 RPG which there are countless of.
(now remember - I said Android TV plays Google Play games - that's a whole differnt way to think of things)
You can get a cheap motherboard and upgrade that later. But you need to know what memory to buy: different motherboards demand different memory.
HOWEVER: To me, it sounds like your excited about i9 and want a motherboard which "is i9 6GHz compatible". That'll be a newer board. It'll cost more.
Pay a little more for the one with Iris Xe 3D: and you can play really good games. (not all great games require nvidia 9000 GL)
Why is also this: once you later upgrade the video card (if you bother), you'll have a backup video card incase ubuntu takes a dump on your other video card. never good to have a blackout - good to have a backup video card incase one has "problems with your setup" (or if you change OS and have new problems) you can try the other. That's why since Intel has made CPU with graphics I always go for it.
"Core i9-features integrated Intel UHD 770 Graphics or Iris Xe (faster). The processor is compatible with Intel LGA 1700 socket "
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all that precludes you jumping into the Arm arena and doing (arduino developer things with a fast arm compiler) (gaming isn't good there i hear). there are just too many choices today aren't there?
me: gaming and Debian don't mix. you could be in for months of frustratoins. google "Xorg black screen", read a few of the hundred million hits.
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