The thing about buying a bike that is loaded with v-brakes is that you will have to double check the hubs, and fork to see if the compatibility is there. When I bought my 2008 hardtail, it’s fork had the IS (International Standard) adapters meaning it was disc ready, but the hubs were not, so I had to replace the rims with a pair that did. I then installed them (rims and brakes) myself, but unfortunately after the cost of replacement of the rims and brakes, I spent much more than I would have if I bought the disc version of the bike and just replaced them with the BB7’s. So look around and see if you can find information through the manufacturers website, or just ask at a LBS.
As for installing mechanical disc brakes, it’s pretty straight forward for somebody who is well, mechanically inclined (or at least handy).