If you don’t want to upsample to more than DSD256 in one zone (and do PEQ in another for example) then a late model i5 will be fine. See the CPU usages in this thread for some comparisons, particularly the gear and setup that Rene (@RBM) is using.
If you have any designs on upsampling to DSD512 then a late model i7 will be a must to even start thinking about it. Roon doesn’t yet support upsampling to DSD512 but it may do so within the life of the server you are about to acquire. My horse n’ buggy (Haswell) i7 coughs and splutters at DSD512.
An i7 is likely to have fan noise that may be disturbing if you are locating the NUC in the same room as your speakers. If all that you are interested in is DSD256 and below, then a good quiet i5 will be a better solution drawing less power and making less noise.
The life of an M.2 SSD is measured in writes rather than time on and it won’t be adversely affected by leaving the server on. This is an interesting article about HDD life. Keeping the HDD spinning will affect lifespan. The moral of the story is to get a moderately priced SSD (SATA rather than M.2) for your music files if possible.