The expensive tools like the blue bosch or the makita/dewalt are excellent tools, that's for sure but probably massive overkill for your dad. These tools are designed for professionals (i.e. used to make money) and to be used all day every day and take the abuse. Really you'll be fine with pretty much any sub £50 drill as long as it's 18V or more, and comes with 2 batteries. If you speak to most professionals, they'll likely have multiple drills, and quite likely that most of them will be cheap as possible drills, on the principle that use it, abuse it, throw it away, charge it onto the job. i.e. disposable. Of course they'll have at least one quality drill, becuase sometimes you really do need it.