Okay, so let's address each item separately.
Regarding the processor: it is true that compiling does use the whole processor. That said, unless you expect your workload to get significantly more intense, if a 6700 takes 20-30 seconds, the i7 will demolish that task nearly instantly. I'm not sure the gains you would indeed get from the i9 are worth the money. On the other hand, you are buying this once and if it will last as long as the 6700 one did, I would say to go for it. (The link you posted lists it as $214 more.)
RAM: the link you posted is compatible. I don't know if it matches the RAM already inside and if there are two more DIMMs (RAM slots) empty, but if it doesn't match exactly, you should take the old RAM out and use only the new set.
SSDs and RAID: My understanding is that if it has two M2 slots, setting up RAID is a software issue. But DON'T get that SSD - it is quite possibly the worst SSD to buy. (Believe it or not, it is actually the first review I ever read about an SSD that turned me off ever buying it.)
Anyway, the drives Dell is selling are "Class 35," which I interpret as NVME 3.0. If you can, ask them if the computer supports 4.0 or not on BOTH M2 slots (sometimes only one supports the latest spec). The fastest 4.0 drives run around $100 each (the slowest that actually function at 4.0 speeds - unlike the Kingston you found - are about half that). You can upgrade to 2TB for $10ish more, so that makes more sense if you are buying the fastest ones. Get a heat sink for each one.
When you figure out if you are buying 3.0, 4.0 budget or 4.0 ultra fast drives, I'll find you a good one.