best iMac for photoshop

Regardless of what type of iMac you get get one with as large as SSD as your budget can afford. Also you've been give excellent info on extra RAM and how it's more expensive to order it thru Apple.

Working with large tiff files adequate memory is important. If you do some extreme Photoshop editing which require large CPU resources a higher rated CPU will help.

Here are the CPU speeds for 21.5" and 27" new iMacs for comparison.

If you want a 21.5", please get one with 32GB of RAM and the largest SSD you can possibly afford. When buying a 21.5" iMacs NEVER buy one with a HD (including a Fusion drive) as that is the performance bottleneck on any computer. Also, the 21.5" models do not have user upgradeable RAM. As a student you are entitled to student discounts. It is wise to buy from Apple's online store and not an Apple Store as the Apple stores normally do not have a wide a choice of configurations. However, if you want you can order from Apple's online store and have it delivered to a store that is fine.

If you purchase the 21.5" iMac be aware that RAM is not user installable after purchase. I'd stick with a 27" iMac which has a door in the back for easy access to RAM/ Also given the price Apple charges for RAM it may be a less expensive option if you purchase it with the minimum RAM and then add the additional RAM after purchase.

Know that Macs are fussy about RAM. Both Crucial and OWC are reliable vendors with high quality RAM.

Unfortunately the best performance comes with the larger 27” iMacs. The 2017 models offer good savings, specially on the Apple Refurbished Store. If your budget limits you to the 21” iMac, consider the 6 core i7 processor over the i3 and i5 alternatives. The 6 core will handle filters and multi layers much better.

You’ll ultimately want 32GB of RAM, but you can get just 8GB and buy additional RAM from vendors such as OWC and Crucial much cheaper. The RAM is not easily installed by end users that do not have a comfort level withi disassembling major parts of the computer. If your budget permits, get it factory installed.

I don’t believe you have a choice of graphics/video cards in the 21” model and that ultimately will contribute to lack of speed with some filters and tools in Photoshop.

Internal storage will require the fastest SSD you can afford. The Fusion Drive, while larger, will really slow down with multi-layer files and large TIFF’s. I would look at fast external SSD Thunderbolt 3 drives for storage and scratch disk for Photoshop. A fast 7200RPM spinning drive will be good for long term storage and archiving projects you’re not currently working on. The spinning drive could be USB3,.

The larger 27” iMacs offer better graphics cards, and faster i7 and i9 processors.

I purchased a 27" iMac for my wife about a year ago, equipped with extra RAM. Some months ago I got myself a 21.5" iMac, also with extra RAM. There is no doubt, her computer is much faster. You will need speed for what you will be doing, especially in the future as Adobe's software becomes even more demanding. I recommend you spend the money now, get a new computer and get what you need. Otherwise, later, you may be kicking yourself. Also, be sure to get a good external SSD. Their prices have really come down in recent times.