If you are limited by budget, just get the 3600 or 3600X, slap a 30-50USD cooler(some people may find the stock cooler noisy) on top of it, and that should give you more headroom for a 2070 Super.
What does the 3700X offer you that the 3600/X can't do?
Or should I go used and get a 1080ti?
If you can find one for under 380USD, and the seller has a good reputation. More than that would be ridiculous. The card's almost 3 years old, and shouldn't be selling for as much as - or more than, the MSRPs of the 2070 Super, nor the RX 5700XT, 2 gpus of which the 1080Ti tends to trade blows with.
Oh, now this is funny. I'm checking Pcpartpicker, and it has currently listed the 4 worst aftermarket RX 5700XTs below 400USD! XD
They all got slammed for terrible cooler designs, so do yourself a favor and avoid those if you're still interested in this gpu.
I want to play video games at 1440p and maybe some 4K too
4K is a whole other ballpark, that I'd recommend a 2080 Super or 2080Ti so that you're not having to drop graphics settings too early in - not that 4K doesn't already look pretty...
I'm just letting you know, if you like high refresh rates, the 2080Ti can't even do it at 4K until low settings.