Any computer geeks out there?

As above, any new computer should have an SSD, and min 8GB RAM. Given this is a photo site, you'd be sensible to aim for 16GB minimum.
My current 'rig' is around 4 years old now, and was second hand when I purchased the base unit. It's running 2 SSDs, a 2TB HDD, a new graphics card, an i7 processor and 16Gb RAM. Cost me around $1,000 NZD all up, which is around ‎£600, and it's getting old now.

Time marches on, and minimum specs increase. It's normal. The K-1 is over a year old already and the K-3 nearly 4 years now.

I get that you've set your heart on an iMac but if you're worrying over the cost of RAM upgrades from Apple then I have to question if its the right choice.

Sounds to me like a well specced desktop PC would be better suited. You can in the future expand the RAM, storage, graphics, whatever you want. You said you're using Lightroom too, which is cross platform.

I'm not a Mac hater by the way I actually use one every day by choice, they're great machines, I love my Mac Book Pro, but anything that demands real power (gaming, photo editing, 3D modelling) I go straight to my PC.

If it's power you're after, you're going to have to pay through the nose for it with a Mac.

Not worrying so much as not spending money where it may not be necessary. I am going to imac because I really don't like windows 10 and the imac just appears to be designed for the professional user. I have just downloaded a trial windows version of the 2018 software I use for work which I will buy for the imac and bizarrely it does things quicker on my old machine than my 10 year old software. So I am pretty much decided on a 16 GB RAM with 1TB memory.

Is that a 1TB SSD? I personally wouldn't bother with a machine that isn't SSD these days. I'd rather have a small SSD (256Gb) and then buy a cheap 2TB External Drive for storage.

fyi, 'memory' is RAM, not storage space!

Slip of the tongue. Standard storage is 1TB fusion drive, SSD is available up to 256 GB. I have no idea what they mean so just googled and didn't understand any of it never mind the advantages of either. I use external storage for work archive and another for my photos so the computer storage is just for current workload. I am sure the guy in the computer shop told me a while ago that the fusion drive is a good thing but now I am confused!

Fusion Drive is 28Gb ssd combined with a 1tb HDD, it appeares to you as 1 x 1Tb Hard drive.

MacOS works out what you use most often and moves it on-the-fly to the SSD so it loads quickly.

The disadvantage is that you don’t get to decide this, and the SSD part is relatively small. The advantage being that you get the 1tb space and for the most part it will appear nice and quick.

I would prefer a 256 GB SSD, and then invest in a large external HDD, but it is down to personal preference.

Whatever you go for, you won’t be disappointed, you’re getting a lovely fast machine either way, you just have to work out how much you value the faster read/write speeds vs internal storage space.

Out of interest have you ever used a machine running on an SSD vs HDD?

I think I have one of each, an Iomega desk unit and a 1TB Seagate portable for my photos but neither say what type they are, neither does the Seagate website. Both run without any noticeable delay. It is really hard to find simple unbiased information on the technical stuff and the manufacturer's websites just give basic specs without explanation.

I would definitely go for an SSD drive rather than the standard 1TB fusion drive. The 1TB fusion drive only has 28GB SSD as paulcliff noted, that is very little to help improve performance.

I would go with the largest SSD drive I can afford and then as prices of external SSD drives are decreasing very quickly invest in a larger external SSD drive at some point.

IMO the internal SSD drive is both important from a performance point of view and to ensure longevity of your iMac.

I have a 2011 iMac and it was becoming slow, so last year I invested in an internal SSD drive and it was a change from night to day.

And another vote for ordering your iMac with the min memory from Apple and then adding memory on your own, it is really very easy and a fraction of the cost. the iMac 27 has four slots, of which 2 occupied. You can place another two memory modules in the free slots.
Just remember to add 2 modules, AFAIK Mac memory goes in pairs for performance reasons. So you can buy 2X4GB or 2x8GB or 2x16GB

This all assumes it's a 27" model

I need faster frame rate than currently specced

With a PC just adding two extra pieces of memory to the existing two is unreliable. Your supposed to sell the existing two and replace with 2 or 4 matched ones.

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That is not the case with Macs afaik, they should be in pairs, but they can be different pairs. That is what I have atm

I would have thought Mac hware was similar to PC's. From Corsairs site regarding PC's

PASTE: 14) I have two Corsair Memory modules that work fine together but when I add two more modules of the matching part number, they won?t work.

Each kit of Corsair Memory is tested to run at its advertised settings in the configuration in which they are sold. If you purchase a 4GB kit (2x2GB), then we can only guarantee its performance and compatibility when using it as a single kit. To ensure the best results with more than two memory modules installed in the same system, it is best to get a kit which has been tested to run in that configuration.
While it is definitely possible that two kits would run together in the same system, there is a chance that you may need to reduce the speed of the memory due to motherboard chipset limitations when populating more than one memory module per channel on the board. If you have problems mixing sets, it is suggested to manually lower the frequency of the memory.

Also from the Corsair forum....
"Thinking about combining 2 or more sets of ram?? even the same part# is not suggested, recommended or supported. "
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Warning for anyone using 32bit Windows the RAM Limit is usually 4GB

Even 64bit Home Basic 7 can't go beyond 8GB

https://msdn.microsoft.com/en-us/library/aa366778%28v=vs.85%29.aspx#physical_memory_limits_windows_7
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I have recently built my new rig and already considering more ram after playing for Lightroom photo merge/hdr. Doing a 4 photo HDR Lightroom grabbed 14Gb.

If you interested my spec is.

AMD Ryzen 1700x (8 core 16 threads)
16Gb 2400 DDR4 RAM (considering another 8 or 16Gb)
256Gb M.2 (Samsung Evo)
512Gb SSD (or fun stuffs ie games
2* 2Tb spinny disks (One Western Digital Black - Photo Storare/Docs, One WD Green internal backup)
Currently Windows 7 Pro 64bit

Future additions will be a second M.2 drive for 'live' photo/video editing etc
AMD Vega graphics