The new 27" iMac's do have the same access door in the back as the last model, as seen here: iMac Intel 27" Retina 5K Display RAM Replacement.
Upgrading the PCIe SSD requires almost completely tearing down the system as seen here: iMac Intel 27" Retina 5K Display SSD Replacement. So far we don't know if you buy a HDD mode only if the PCIe SSD slot and the sundry parts are even present as Apple has in the past has not populated the logic board. This is the case in the 21.5" model as you can see here in the IFIXIT teardown:: iMac Intel 21.5" Retina 4K Display 2017 Teardown
So I would recommend you get the PCIe SSD based system and I also recommend you get the largest SSD you can get as Apple is still the only source for their custom SSD..
Then if you want to add more storage (HDD or SSD). You can then add in the SATA drive later.
Just keep in mind these 'Thin Series' systems are a bear to open so make sure you follow the IFIXIT guides to the letter and use the correct tools as you can easily damage the display in the process of taking the display assembly off which would be a very expensive opps!
The term 'Hard Drive' is passé, instead we use terms HDD or HD for the older traditional spinning disk drives or SSD for the solid state drives. If you don't know which then just use the term 'drive'.
So yes, its best to get the SSD only model or the Fusion Drive model which has both a HDD and SSD fused together as one logical volume (two drives seen as one).
As to the PCIe SSD its self, Apple does not sell it as spare (only as an exchange of the same size) and the parts houses which buy compete systems and tear them apart for the parts charge premium prices! So get the largest PCIe SSD you can afford now.
As I stated if you got the PCIe SSD only system you would still have a spare SATA drive bay (3.5" is what we expect here until IFIXIT does the teardown to verify) which will allow you to add in your own drive which could be a HDD or SSD. But! It's still a bit of work as you'll need to get the proper SATA/Power cable harness from Apple and as you are putting your own drive in you'll need this: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later as unlike the custom Apple HD drives you need to add in a thermal sensor for the system to work properly.
If you get a Fusion Drive'd model you will have a dual drive system so you would need to swap out the SATA drive to a larger one if that was your direction later on.
Frankly, Apple is really making it harder to do any drive upgrades these days with the 'Thin Series' iMac's. If you can, get what you want now for drive storage.
Given the risks of damage and the amount of work to even get into the system it's not something I recommend for someone who has not opened one of these newer 'Thin Series' systems before.
Sorry Simon didn't see your post.
Then you want the largest PCIe SSD model you can afford. It will be the fastest! Great for running your OS & App's and hosting the OS & Apps cache & paging files. Then use an external Thunderbolt HD or install your own internal SATA drive (dual drive config).
While a Fusion Drive is in truth a dual drive setup, Apple does not offer a very large SSD so it may not offer the speed you are looking for and even if you broke the Fusion Drive set to be discreet drives you might find the limited space of the SSD won't be enough to hold your OS, Apps as well as what the OS & Apps need for cache & page files.
Thanks Dan!! Appreciate your reply and the answer is helpful :)
door Simon Yong