What considerations are there around replacement of cable-actuated disc brakes with hydraulic-actuated calipers? I have a Dawes Discovery 401 hybrid bike that was fitted with QuadQMD-6 mechanical disc brakes.

I like the bike but the brakes are beginning to show signs of wear and direct replacements are not available. So I was thinking of replacing the calipers with hydraulic ones and converting the cable-operated system to a hydraulic one.

I'm pretty handy with a spanner and would like to try this myself.

Is it easy to convert from cable to hydraulic and what bits would I need for adapting the cable runs? Is there anything else I need to consider?

I don't want to spend a fortune but would like a decent set.
Rotors: 160mm
Mount: post.
If the brake lever is integrated with the shiftier, you will need a new shifter. 

Cable run for Hydraulic will be continuous, and you wont have C-Clip mounts. You will need to use zip ties to hold the cable to the frame. Functionally there is no problem with this, how you feel about this depends entirely on if you prefer form or function. 

MTB Caliper mounts have been standard for 15 years or so (Road has a different standard introduced by shimano in 2010 ). In most cases you will be able to unbolt the mechanical calipers and bolt on the new ones, however some calipers are aimed as a different minimum rotor size. This means you may need a new adapter depending what you have and what you get. As long as the new caliper minimum rotor size is not larger than the rotor you have, you can get an adapter to fit.   

I would think about installing new rotors (although $$ is a consideration). The old brakes were not expensive, so the rotor won't be the best quality, new rotor would keep everything matching.  It may make no difference but might help with longevity and reliability (e.g. if they are warped).