16 GB of RAM WILL future-proof you. I'd say 8GB is more than enough for general purpose use over the next 10 years, however. You don't need to denote the drive type, because it's a given with the iMac; what you need to decide is how much, since you will not be able to upgrade it. Even for general purpose use, I'd opt for at least 256GB of space, even if 128GB is offered (it might not be).
Personally, I have a ten year old CPU powering a two year-old GPU just fine, since it was nearly to-of-the-line back then. Along with the MB and CPU, the 12GB of generic RAM are original. the HDD was replaced with a $100 Samsung 1TB SSD and the second slot was populated with a 2TB WD Black HDD for backup.
I wouldn't even consider an iMac.
16GB is probably the least you should get if you cannot upgrade (soldered on) the RAM, and SSD would need to be at least 512GB if you want future proof (many people don't want to backup and migrate). You can get a slower CPU however, the difference between top of the line and slower CPU for non workstation workload isn't really that much, 4 cores is all you need and I think most Mac are pretty powerful and they don't sell the bottom barrel stuff like PC does.
Still, all it takes is 1 hardware obsolete to make a machine obsolete. Video codec change? New AVX instruction? New CPU due to security flaw? Too bad, everything must go. Buying cheaper hardware and update more often (and sell the old machine), is the better way forward.