I didn't set the task to do a polygon, but if there is such functionality, I decided to try it. M1 and WP got up well and they have enough of everything, but with M2 there was of course a problem. I watched miniITX or microATX, but I would not like to be engaged in a constructor. The 920+ has a lot of other things, and this was an advantage, from the size of the box to the functionality and noise from work.
Yes, here the ceiling around the M2 will have to be put on the computer separately on MAMP PRO.

nginx+php+mysql
where exactly are the difficulties?

M2, perhaps, will be more voracious than Bitrix.
If you get RAM up to 8 Gb, you will run into the processor.
The minimum hardware is written for M2:
RAM of at least 8 Gb.
CPU is at least 2-core, with a frequency of at least 3 GHz.
And you have Celeron there.

It is bad to count other people's money, but if you have such a NAS, then I will assume that you are not straitened in funds.
If there is a desire to have a testing ground at home for experiments or home hosting,
then it may make sense to assemble a compact and relatively inexpensive system on miniITX or microATX.
On the same Pentium (LGA1200) 4GHz instead of 2GHz you have, or even Core-i3 as needed.
With an SSD drive, she will eat 70-80 watts at the peak, and the average consumption will be around 40-50 watts.