aledowenthomas

Just thought I'd let you all know that I purchased the bog standard 27" iMac. It runs Affinity Photo like a dream with no slowing down at all and obviously it's way faster than my old iMac. When I get a new camera with larger files then I'll look into fitting more RAM, but as it stands there's no need. 

Hi, I am just working on a new standard iMac 27 inch (2019 model quad core) with 8 GB and fusion drive. It works very fast with al the new Affinity products.
My old Macbook Pro with SSD drive and 16 GB do also a good job but the iMac is  faster. (I mostly work in Affinity publisher and Affinity Photo)

Which one you get will depend on your budget & how far up the price/performance curve you can stretch it to go. Higher performance is always better & a bit more future-proof, but as you go higher up the curve, you get progressively less of an improvement per dollar spent.
Fusion drives are faster than regular ones but SSD's, particularly the ones using the fast multi-lane interface in the new iMacs, are much faster. Personally, I think the 256 GB SSD upgrade is probably too small to consider, so I think the extra roughly $270 US is worth it if you opt for an SSD. The 1 TB SSD would be nice but you could buy a much larger than 1 TB external USB 3 drive for the $180 US cost difference. That's what I would do, anyway.
Regardless, I would not pay Apple's prices for more than 8 GB of RAM. Factory installed modules are screened to tighter specs than 'budget' third party ones, but reputable third party sellers like Crucial also guarantee their modules meet Apple's specs. A quick web search will show you what Crucial is asking for modules compatible with the latest 27" iMacs. Compare (for example) what it costs using four 16GB Crucial modules to max out the RAM to 64 GB (probably overkill) compared to what Apple is asking for just a factory installed 32 GB memory upgrade. Even if you just throw away the two 4 GB Apple modules of the base configuration & opt for four 8 GB Crucial ones for the same 32 GB total, it is much cheaper than the factory option.
Note: Crucial is just an example of a reputable seller. There are others like OWC (& they will pay you a little for your OEM modules if you buy new ones from them) so shop around a bit & find a seller that works best for you.