It's hard to say what a "hobby" photographer is going to need. Some hobbyists get involved with rather complex post processing that can challenge the most powerful current iMac.
I just replaced the the machine that I use at work for graphics/image production. I replaced an ageing Early-2009 PowerMac with a new iMac. Since I regularly work with 36mp RAW images in Lr and images in Ps so large that the PSD format won't support them (I have to use the PSB "large document" format to support 48x96" 300ppi images with 1-2 smart objects each with 1-3 Filters and 5-15 other layers and file sizes from 5 to 12gb), I chose to go with a fully blown iMac 27" 5k with the i7, 32gb RAM, an a 1tb SSC. All of the image file live on external drives (10tb total for now, but another 8tb will go online this summer). The new machine is noticeably faster than the old PM (dual processor 8 core, 16gb RAM, 7200rpm HD).
If you work with massive files like I do at work (I work for an art photographer) then you need the RAM, the SSD, the i7 and a whole stack of external drives. For more modest print needs and for web resolution images a more basic i5 with 8gb should do perfectly well, and RAM can be upgraded later. You'll want either a larger than default fusion drive or, if you go with a SSD, expect to need an external drive for you image files at some point. At home, I work with a Windows 10 box with an i7, 7200rpm HD, and only 8gb RAM and have no issues with my personal Ps & Lr work, even with print sizes up to 13x19.
Future proofing is difficult and can be expensive. You might consider opting for the i7 since the processor can't be upgraded later, but for modest sized RAW files and even the highest resolution cell phone images the i5 should work fine.