I think you have options before welding. A2Z do a disc adapter that you can fit without welding, though you have to have the right drop outs. Specialised also did their own adapter that fitted between the drop out and the v brake boss. Try googling brake therapy as well, as they did a similar thing. Failing that you could get one welded if the rear is steel, not so sure how easy it is for aluminium though due to heat treating etc.
If you go down the disc route from v brakes (as I dis once), you'll need at minimum new wheels a d disc brakes, possibly shifters too if you have sti lever/shifter combos. Cable routing may also need to be looked at, either drilling out existing cable guides, or getting adapters depending on your preference. I assume your forks have disc tabs, otherwise they'll need to be replaced too.
All in all not cheap, and the benefits are really marginal in my view.