OK lets start from the top:
Adding RAM - While you can do this its a very big job as you'll need to remove the logic board to gain access to the RAM slots which are on the back side. Review this IFIXIT guide: iMac Intel 21.5" Retina 4K Display (2017) RAM Replacement.
Adding a HD - This is tricky!
- If you have gotten a SSD only model then all you'll need is the cable and rubber cushions.
- If you've gotten a Fusion Drive model you'll end up throwing the HD way that makes up half of the Fusion Drive set, and the SSD which is used with a fusion drive config is only a 128 GB so its on the small side unless you re-create the Fusion Drive set with the drive you would be putting in.
- If you have only gotten a HD only model then you will not have the needed mountings on the logic board for the PCIe blade SSD.
Here's the IFIXIT guide for the HD: iMac Intel 21.5" Retina 4K Display (2017) Hard Drive Replacement and here is the IFIXIT guide for the PCIe SSD: iMac Intel 21.5" Retina 4K Display SSD Replacement
So in a nutshell I would recommend you get the largest PCIe SSD only model you can afford for now, forget about the internal HD (or Fusion Drive). As far as RAM: Pay for going to 16 GB now so you don't need to open the system.
Instead put your money into a good external Thunderbolt RAID 0 HD system. It will be faster than the slower internal 2.5" SATA III 5400 RPM drive Apple uses. If in a years time, you find you truly need more RAM for what you are doing, then do it (basically wait until the warranty expires before opening).