I am in the market for a 27inch iMac. I am replacing a 2000 21.5 inch and looking forward to more real estate. I cannot afford the top of the line. I will opt for SSD and probably 512GB or 1TB. Other than that I could use some guidance on processor and other bits and pieces. We are a family of educators and technologists - who take photos, listen to music and build Web sites. We also do the usual with Omnifocus, Mail and Busycal.
What recommendations and thoughts have you?
Neil
Where are you, and what is your budget? And what specific bits and pieces are you wondering about?
As for processor choice, we did discuss this here.
Above link is broken
Hey must have taken it down. The gist was when building custom orders, make sure you don’t cross the line into the next standard SKU
It’s a general rule that the entry systems give the best bang/buck (in other words, options give a decreasing rate of return) providing that the system will “do the job”. Also processor upgrades give a particularly poor return. Now days you definitely want an SSD, especially since the new APFS is designed for them and seems to perform particularly poorly with spinners. If you are buying a 27", buy any additional memory elsewhere (like OWC/macsales.com) for much less. Takes only a minute to install.
The only thing you are doing that might call for more processor power than base line is photo processing, but that would be if you are heavy into Photoshop or similar and have a high resolution camera, and are in a hurry.
Is the memory thing still true? I got the 27" and I don’t see the access door on the back any more. I stopped at 16GB and will probably like to upgrade later if that is still possible.
It’s there.
I would say: choose a 27 inch model with 16 GB of RAM and at least a 512 GB SSD. Checkout refurbished models.
Concerning the processor, you will be fine with an i5. If an i7 is feasible budget-wise, go for an i7.
Concerning the RAM: it is perfectly fine to buy a “cheap” 8 GB model and to install more memory afterwards. It is very easy, like @bowline already has stated. 
Thank you very much!
Thank you all. i5, 512K, 8G is where I am heading. I see several i5s with different processors! How to choose?
Well, there are several things involved:
a quad-core CPU is better than a dual-core CPU, a 6-core CPU is better than a quad-core CPU
after that: the more GHz the better
a 9th-generation processor is very new, a 8th-generation processor is a recent processor and a good choice, a 7h-generation processor is absolutely ok in a refurbished iMac
How a processor performs can be seen in benchmarks:
https://browser.geekbench.com/mac-benchmarks
I looked at the 3 processors (CPUs) available. The machine with the bottom end CPU has a lower version of the GPU (graphics card) and is limited to 32GB memory, 2TB Fusion drive or 1TB SSD at purchase.
The next machine up has a very small performance upgrade on the CPU but more built to order options - up to 64GB memory and up to 3GB Fusion drive.
The top end machine has a higher CPU performance and an option for a much better GPU.
I’d say bottom end machine if the available memory and disk options are suitable for you
Top end machine if you need that GPU.
Don’t get a Fusion drive, get SSD.
If you plan to upgrade memory after purchase, verify that the model you choose allows this. It’s not clear from the Apple site if all 3 models allow memory upgrades.
For what it’s worth, after 3 iMacs, I switched to MBP. I got frustrated with the amount of desk space taken up by the iMac, the fact that I couldn’t use the display for other non-Mac machines and I couldn’t take it around in my backpack. But the iMac is great value for money otherwise.