I am not entirely sure why there is so much obsession over concerns about a used bikes not having a warranty. Surely that is a given which comes with the territory. All my bikes have been purchased second hand over the years and drawing particular attention to my summer road bike I purchased this for around £300 when the prevailing new model in the shops was £1,000. Even if some catastrophic failure had taken place I could have gone out and purchased another used bike and still have been £400 better off. However in reality most mechanical problems on bikes are not catastrophic and can be fixed using a couple of hours spare time and a Youtube video, or at worst by your local bike shop.
In my experience a decent pair of well fettled rim brakes are more than good enough if you are only planning on fine weather riding. Whilst using my 105 calipers I have never even applied maximum force to the brake levers because I start to head forward of the handlebars before I can test full stopping power.