using a Fiio Q1. i would recommend this. at this price this is hard to beat. Can consider Q1 Mk2, too.

yes, Q1 has 2 inputs, micro usb and 3.5mm;
1) phone's usb otg (OTG is a dongle from micro usb to normal usb) to Q1's micro usb. For phones that uses usb-c, you can get usb-c to micro-usb cable from aliexpress. Aliexpress also can find short micro to micro.
2) phone's 3.5mm headphone jack to Q1's 3.5mm input.

what phone are you using?

Q1 has build in battery, and you can switch on the charging function as you wish when connect to usb, and you can still use it when charging over laptop.

K1 doesn't have a battery, so it will draw power from your phone or computer when using it. it is smaller and lighter than Q1, but Q1 can power more power hungry headphone, simply because it has its own power source. As for reliability and durability, both are equal, i think, as both are made mostly out of metal. Although Fiio is china brand, they are quite well regarded in terms of build quality.
depends on your usage. Q1 is a DAC/AMP combo, can connect via USB and 3.5mm
A5 is pure amplifier, people usually use it with a DAP rather than a phone. If chip to chip compare in amplifier department, A5 should be better.
Maybe you can audit it? Starspicker seems to be carrying  both

Q1 is the best option, long term wise. We will upgrade our setup in the future; so, it doesn't hurt to have a decent setup now 

K1 would be good if you dont plan to upgrade past m50x, as m50x is relatively easy to power.
If you plan to upgrade to something better with higher impedance, Q1 would be a better choice for future-proof, as it can power a lot of headphones, even 300ohm Sennheiser HD600