Another vote for Shimano hydraulic discs - one of the company's better products.
They come pre-bled, ready to go.
Hose lengths are the same on most bikes, so it's very unlikely you will need to alter that.
Given the vast majority of braking on a bike is done at the front, a cheaper and simpler option would be to put a hydraulic on the front and leave the back as it is.
One thing you will need to work out before buying is whether the calliper is post mount or side mount.
Other than that, the brake should bolt straight on.
Although as we are talking about working on a bicycle, there might be a little snag or two to overcome somewhere.