dell is NOT a trustworthy pc vendor, so easy enough option 1 is gone.
if you don't want to buy from ebay, option 2 is gone.
you can buy hp, asus, msi, or any custom laptop from a specialty dealer near you that customizes laptops.
if you buy hp, msi, or asus you may be able to do the memory and storage upgrades yourself.
if you want some sort of future proofing for external devices, thunderbolt will allow for that. the new laptops with thunderbolt will be a bit more money. the asus ROG G752VT-DH72 is an example of one of the new laptops with thunderbolt, that you could upgrade memory and storage yourself. you can download manuals for any laptops you are interested in, to see what upgrades are available. if you don't want thunderbolt, last gen laptops with i7-47xxxx or i7-48xxxx are very similar in performance, but can be alot cheaper.
either of the cpu's you listed are going to be good for a laptop, but the i7-x8xxxx will be a bit better.
for premiere you really want to stay with the gtx 970m or better. 4k with alot of gpu effects and lumetri may need a gtx 980m or gtx 980.
finding a laptop with 4 memory slots will allow better upgrades, as you may want to have 24-32gb for AE.
if you upgrade/add a m.2 ssd, the samsung 950 pro m.2 ssd's are really good for media and cache on one drive, if capacity allows.
samsung also sells usb 3 ssd's if you need more drives than the laptop you get will hold, or to quickly move projects between computers.