Will a Mac Mini work well for Graphic Design?

I agree with Jimmy that i7's are the way to go, but in my experience Fusion drives are not much faster than standard drives, nowhere near the rate at which SSD's can move stuff around. Especially with large files.

the ones I have here with fusion are pretty fast but in retrospect I would have done SSD to boot and applications and external USB 3 for the production system storage/TB2 for the video editing system storage.

if you can do a small SSD and store assets externally it might give you flexibility in putting more cash towards something else you need at time of purchase (CPU/GPU) and something you can expand later (RAM/Storage)

Not very easily, but there is nothing stopping you from connecting one externally, it will be limited to the speed of the connection method, some of which are much faster than the internal connection.

I gave myne a boost to 8G memory, ( mid 2011) and beta testing TurboCAD Mac and Shark CAD programs have been smooth. I'm still running Camtasia 2 for vids, and those are viewable @ You Tube> John Borlaug for some examples.

Get good enclosures and install your drive of choice, rather than buying externals from WD or Seagate. OWC (macsales.com) sells reliable enclosures for external drives.

I am not a fan of iMacs, but given Apple's lack of focus on people who do actual work on actual computers, it might be the best you can do right now. I'd be more inclined to get a used 2012 or 2010 Mac Pro. Very capable and affordable.

I will take your word for it. I really hate the whole iMac concept, which seems to get less user-friendly and more wasteful in terms of recycling with each iteration, but lots of people are stuck with it because there hasn't been a worthwhile tower since 2012.

My point is if anything the iMac was a far smarter investment then either going new Mac Pro or throwing money into whatever we could update on the 2010 Mac Pro's - and we got killer displays on top of that.

CPU advances have slowed down for the last few years (yes, I know AMD announced a 32 core chip but it's like $500 for the 8 core models - then the $$$$ for that logic board? - Oy!)

SSD's are pretty straight forward tech and not likely to change with only smaller speed gains and more likely more storage for less, the price and storage of SSD's is becoming more consumer friendly and the technology is usable on computers made well over 10 years ago

We are starting to see more DDR4 RAM on logic boards and moving away from DDR3 except on all Mac models at this time.

GPU's are becoming more powerful and less expensive in a consumer friendly way.

GPU is where most of the consumer advances are going now but primary development is focusing on games with many of the professional developer tools promised with these cards is delayed while the game features get choice development, but along with that is the software that can utilize these tools and many older software packages would not gain much (or any) performance from these newer series of cards because that software can't utilize the technology they support, or they support technology from another series of cards for professionals with insane price tags or they support one platform and not another (e.g. CUDA from Nvidia, or AMD Pro Render which don't work on the competitors devices or older cards from the same companies who made them.)

Any computer is only as fast as it's slowest component, sometimes that component is the software that you choose to run, not just the computer.

A display replacement would not require a logic board replacement, a GPU replacement would.

The cost of Apple care which comes out to $0.14 a day would cover the entire system for three years if you didn't want to be out of pocket for repair or DIY.

Also you can run one additional display at 5K res or two additional displays at 4K res out of the 2015 iMac

the internet is full of mac conjecture and thats all it is. If you were to make a note of half the tech that is cited to "come to Apple in Qx" that didn't come from directly from Apple and didn't happen you could cover the Australian outback with it.

I have a dozen iMacs here and dozens more I've supported over the years for intense production work, to date I have not have had a single LCD failure, and these computers see at least 45 hrs of up time a week. There were too many fail points on the Pros at this time I made and continue to make purchases, but if Apple revisits them I'm open to the next generation, but I wont buy any conjecture until Apple makes an announcement themselves.