I generally buy the stock machine and then build around it. For my purposes I get more "bang for the buck." Also, I mostly go for the entry level machine unless there is value for me in the next rung up. I did not see that in the new (Mid 2017) crop of 27" iMacs, esp. at the $1625 shipped price @ Adorama (w/ Appleinsider coupon). So we start from there. Bought an additional 16GB RAM on Jet.com for $108 shipped after 15% coupon (1st 3 orders offer). RAM is 2x8GB Corsair Vengence 260 Pin SoDIMM (DDR4 2400 (PC4 19200), CAS 16, 1.2v, Unbuffered. I was going to pull the stock 8GB (Micron) but decided to test the machine with the 8GB plus the additional 16GB. No problems, running at full speed so I left it in. I have 24GB RAM now for just $108 additional. Ecstatic about that. H/T to 9to5 Mac on that one. One of the guys there bought the 32GB kit and did a write up about it and also posted a video. For my boot drive I looked at the usual "budget" SSD suspects: Mushkin, SanDisk, Crucial. (Did not consider Samsung... to expensive for my needs). Decided on the 1TB Mushkin Reactor as it had the best overall professional reviews and actual user reviews. The Reactor is an "older" MLC model but it has built-in garbage collection. $270 on Amazon (Prime). Finding a USB 3.1 Gen 2 enclosure for it was quite the hunt. Not much selection out there right now. I decided on the MiniPro USB 3.1 from Oyen Digital (yes, never heard of them either). $30 on Amazon, prime eligible. Comes in silver and black. I bought the last "Prime" silver but probably more in stock soon or the manufacture can ship to you, also free. The enclosure is aluminum, big plus for me. It also supports TRIM -- though have not tried to enable TRIM on my iMac since the Muskin has garbage collection. It's bus powered and includes both USB-C and C to A cables. Running Black Magic's Disk Speed Test I get an avg 425 MB/s read and 512 MB/s write so I'm very happy. I only have OS X and documents on this drive. No photos, video, or music. Disk Speed Test reports it can handle up to 2K just fine except for the most uncompressed files. Then it gets dog in high humidity slow. I decided this was a good time to upgrade my photo and video drive so I moved over to a 4TB RAID. I bought the AKiTiO NT2 U3.1 (gen 2) enclosure. Again, all metal enclosure. The fan noise is avg. Not silent, not loud. I am not in a quite environment so it very well could seem louder with fewer background noises around. The fan has its own switch though if you want to risk it or put in SSDs. The enclosure supports RAID 0,1,JBOD,SPAN. It was $84 on Amazon. I could not find a less expensive USB 3.1 Gen 2 RAID enclosure for 3.5" drives. To fill the NT2 I decided on 2TB WD Red. The Seagate Iron Wolf was $10 cheaper, so $20 less total, but it ran at 5900 rpm where the Red runs at 5400. I like the slower drive in a small RAID box. Also I'm not sure I trust Seagate enough in RAID, but, obviously, WD isn't bulletproof. That is why you backup. Unfortunately I could not hook up the NT2 to a USB-C port because there are only two on the iMac and they are both in use. I cannot find a USB 3.1 Gen 2 hub for the life of me. If anyone knows where to get one please share. Speed Disk reports an average read and write speed of 227 MB/s. A tad disappointing but maybe not for 5400rpm drives. Reds are not built for speed and I understood that going in. My main goal was heat and let the RAID take care of the speed. Maybe I'll test the drive with USB-C and see what the speed difference is there. Finally, I have my trusty LED Cinema Display. It's a refurb from 2010ish. I love this monitor even though I could sell it and literally get an LG 4K with the proceeds. Of course the problem is it's Mini DisplayPort. OK, no problem. I bought an "Exwin" brand (Is that a real brand? Probably not). USB-C to Mini DisplayPort adapter on Amazon for $22. Works like a charm. I just wish I didn't have to sacrifice a USB-C port for it. So there you have it. Total cost including the iMac: $2112. For me at least this is a better, more useful package than if I just bought a stock 3.5Ghz mid-level iMac for $2K. Anyway, I spend some time trying to ferret out these add-ons, mostly because USB 3.1 Gen2 stuff is still quite rare, so hopefully it can help guide others here.