but after hearing about apple leaving intel i'm concerned that i'll be left behind support-wise in a few years. am i being paranoid, or is there genuine reason for concern?
Even if Apple do shift processors in the near future, there's no reason to think that current products won't get the usual level/duration of "support", especially if you don't desperately need to upgrade to the latest OS
every year. AMD processors basically run the same code as Intel (in fact,
Intel uses the
AMD instruction set for 64-bit) so that would just be a case of who offers the best specs at the time. A switch to ARM would be more fundamental and (ultimately) require software developers to re-build their applications for optimum performance - more like the 68k-PPC and PPC-to-Intel shifts of the past, but in those cases Apple maintained support for years. Frankly, on past performance, it could be worse to be an "early adopter" of any hypothetical AMD or ARM system - I think it was the early Intel Core 1 and 32-bit Intel Macs that got the shortest period of support from new MacOS versions.
I'd say the most likely next "axe" to fall on MacOS support will be a requirement for iMac Pro-style "T2" security subsystems - and that clock can't start ticking until Apple have implemented that across the whole range. (The T2 chip might also be the source of some of the "Apple switching to ARM rumors").
However, if your current Mac is getting the job done, you might want to wait until the new 8th-gen Intel chips appear in iMacs, since - especially if you're doing audio or video work that is multi-thread optimised - the extra cores in those represent the most significant performance boost in years. I think a 6-core i5 could be the "sweet spot" for audio in an iMac. Unfortunately, nobody knows when those will appear (rumours of October but...) or whether Apple will just upgrade the chips or "improve" the design (which could see the back of upgradeable RAM, USB-A ports etc.) - however, if the new chips don't show up in the next few months it could be time to start pricing up PCs...
Definitely go for SSD - DIY upgrading a spinner in an iMac is something to consider only after the warranty has expired - 512G will probably be fine, 256 OK for general use, but may be tight if you're doing audio (A full Logic Pro install is 56GB, 1TB is expensive, but buys you the right to be lazy about disc management.
that said, your external USB option has my interest piqued. don't know much about it it. can you explain the gist?
USB3 external drives come in all sizes and types, SSD and mechanical HDs. A mechanical hard drive with 2-4TB of space for archiving less-frequently used stuff is probably fine for what you want (and more than fast enough for playing back finished audio and video files). Or there are SATA-based 'economy' SSDs that are vastly cheaper per GB than Apple's built-in drives but still far faster than mechanical. If you are actually going to "work" on the external drive, there are more expensive super-fast SSD options like the Samsung T5.
also, does anyone know how much RAM is supported? it seems that 8 GB of RAM is the stock norm, but the software i'll be running (ProTools 12) says a MINIMUM of 16 GB is needed, ideally 32GB or more.
The 27", 5k iMacs have RAM that is very easy to upgrade yourself, with 3rd party RAM costing significantly less than Apple's upgrades (although the prices fluctuate). Unless you see a good deal on an upgraded machine, get an 8GB iMac, get a 16GB (2x8GB) upgrade kit from the likes of Crucial and you'll have 24GB total for less than Apple charges for an upgrade to 16GB - and if you find you need more, another 16GB upgrade kit will take you to 32GB.
