With all iMacs, you will experience fan noise under full load (or when you run a game or a batch export). Whether the i7 will be noisier than any of these i5's depends more on the actual load than on the CPU.
Also, my experience is that there is a thing called "thermal paste lottery" when buying iMacs. If you are unlucky even the base i5 model can be hot and noisy.
As for the storage, my experience is that 2TB or 3TB fusion is just fine for Lightroom (1TB is not). The fusion is doing more or less what you expect it to be doing (offload big masters to HDD while preserving your catalogue and previews on the SSD). It is a fun to watch it in iStats Menus that SSD and HDD are working together.
The FD doesn't break record speeds, however, it doesn't feel slow. You get most benefits of SSD (mainly low latency and unlocking small I/O which is way more important than absolute speed) with much bigger capacity for money, In fact, the Lightroom works quite well even when masters are located on NAS which is even magnitude slower (latency-wise) than the FD. I would rather than a big internal fusion than a tiny SSD + bunch of external enclosures and permanently have to move stuff around.
i7 will reach max fan speed in 30 seconds.
i5 might take 8-10 minutes to get there.
That can be a huge difference in real world usage, because the i7 will hit max fan with just heavier daily usage, whereas somebody with the same usage will never hit max fan with an i5 (particularly the 65 Watt models). Furthermore, this has been a pretty consistent observation. If there is a "thermal paste lottery", that has not proven to be as important to the actual CPU itself, i7 vs i5. Basically on average, judging by observations here, if you have very heavy CPU load:
3.4 GHz i5 - You might be able to run this thing for extended periods without hitting max fan.
3.5 GHz i5 - You can run this thing for short periods (many minutes) without hitting max fan.
3.8 GHz i5 - You can run this thing for short periods (many minutes) without hitting max fan, but it might be a little quicker to hit max fan than the 3.5 GHz i5.
4.2 GHz i7 - Will hit max fan within 30 seconds.