Here's some opinion(s) that you might consider - and this is written from the standpoint of someone who needs to have like 99.9% uptime cause this is my livelihood - I can't have a system go down for a week:
All of the current Apple models depend on external attachments to get anything done.
If you want to efficiently stream samples, you need a reasonable size HD or SSD. For a mechanical hard drive, reliability is an issue so backups even of sample drives are critical otherwise you can spend days reinstalling stuff. Performance wise, mech HD's are ok as long as you don't fill up more than half of the drive space, and as long as you don't dedicate them to more than one task. Read / write performance once the mech HD is more than 50% full drops off approximately exponentially. For example, using a internally mounted mech HD as your system drive, plus streaming samples off of it, plus using it as the record / save drive for your Logic projects, plus streaming video off it (if you are scoring to picture) is totally asking for trouble. You will never be so frustrated as to be in this situation.
SSD's provide much greater performance, and the price per GB of space is dropping rapidly as well. Still with SSD's you need to fill it up only 80% of the way, and at that point performance drops off as well. And even though there is no physical drive head on an SSD (which is both the read / write bottleneck with mech HD's and the moving parts which can fail) you can dedicate it to a certain number of simultaneous tasks before it won't handle the load. So using an SSD for the system drive and to save your Logic projects is probably safe. Even streaming some samples, fairly safe. Streaming an entire orchestral mockup is probably asking for frustration.
So most people are going to use external drives to:
1) stream samples (external SSD)
2) to stream video for scoring to picture (SSD nice, but mech HD will work)
3) to handle backup tasks - as long as you are copying legacy sessions on to your primary audio / Logic record / save drive instead of streaming from the back up drive, mech HD's are very cheap per GB and a mirror RAID setup is a good idea as mechanical HD's fail all the time. I have had 5 go down in the last 5 years.
assuming that you put your system recourses and your logic audio record / save on the internal drive.
All the new iMacs have lots of USB3 ports which are fast connections as well as thunderbolt so you have lots of connectivity and options.
Price?
Any aftermarket solution is going to cost less than Apple - period.
http://www.macsales.com OWC has various options like Paul mentioned for external drives, and they also have had various turnkey upgrade programs to replace internal drives, add memory etc. That used to be the price performer for upgrades.
After fully researching what OWC can do, I would recommend getting the cheapest smallest internal storage with the iMac, and then getting upgrades to SSD's etc from OWC. You will probably spend less.
I wouldn't bother with the Fusion drive - cause its an SSD / flash drive mated to a mech HD, and again, the mech HD can fail quickly. If you don't have backups, you can be left high and dry. Even with backups, if the internal drive goes down, its still going to cost you time.
HTH - I was in your position last fall.
Had an aging Mac Pro 1,1, the CPU wasn't powerful enough to do much anymore with the modern software suites. I considered a lot of different angles - Hack Pro, a new Mac Pro, used Mac Pros, iMacs, Macbook Pro, Mac Mini's both to replace my mac and also as farm computers to stream samples.
What I found:
http://www.everymac.com/systems/apple/m ... acpro.html - I linked to the Mac Pro section but there is a section for every Mac Model. This site has single thread and multi-thread / multi-core performance scores for every mac ever created, and you can get a good idea how your machine stacks up against other similar models. Most DAW's now run with some kind of multi-thread / hyper thread support. Pro Tools does since V11, and Logic has since v9 and Logic X now supports 24 virtual cores if you have a true 12 core machine. Most operations in Logic, and bouncing in real time are going to use multiple cores / threads. Single thread applications are bouncing offline, and doing tasks like web browsing etc.
You will find a wide spread of performance between all the CPU's with 8 virtual cores (like the iMacs, the Macbook Pro's, the Mac Mini's etc) and the Mac Pros that range up to 24 virtual cores. Its hard to know which is right for you without being able to see hands on what some people are accomplishing with a certain build, but the multi-core performance scores give an idea.
Having a lot of RAM helps, but its really just for headroom. Its unlikely you will be running an orchestral template with 30 GB of samples pre-loaded, and even the most robust CPUs are going to freak out under the load if every track in your template is triggering MIDI > samples. I have 32 GB of RAM, and most of the time the load doesn't exceed about half of that, no matter what I am doing. That amount of headroom is fine.
I also realized that like I said above - modern Mac's aren't really built to accomplish a lot of streaming tasks unless you get the flash / SSD drives that Apple includes in the factory builds. But that comes at a major premium (especially for the Mac Pro 6,1 - yikes). Even that is a major question mark - how many simultaneous tasks can you accomplish? How big of a mock up? How many audio tracks?
So I ended up buying a used Mac Pro 5,1 12 core with 32 GB of ram, and I got a great price. Way less than you are going to spend on your iMac. You can search on this forum for other threads where I have blabbed on and on (like this thread LOL) about why I ended up with this machine, and what it has made possible with streaming samples (like I have two PCI mounted SSD's of 512 GB each) and backups (large internal mech HD's for redundant BU's).
Just an opinion - but spending a lot of $$ on the newest and greatest iMac may or may not be a great investment. Arm yourself with as much knowledge as you can.
There are only a few things I believe are "for sure":
1) computers are an expense that you need to undergo to do this work
2) you can write off the expense
3) saving a few bucks now may or may lead to frustration in the future, but you can insulate yourself by getting really knowledgeable about your buying choices
4) most people skimp on backups. I found out the hard way that this costs time (and ultimately money earned).
5) limping along your present system is money in the bank cause you already own it. If that means saving for another year and get something more powerful that has a longer shelf life, that might be a good decision. My first mac pro I bought in Jan 2007 and used it until Sept 2014. My present one I can see using for another 5-7 years.
Good luck!!!
