Having done a mtb-> drop bar conversion lately, one issue you will probably run into is that frames designed for flat bars are longer in the top tube and shorter in the head tube than frames designed for drops. If a frame starts out fitting you it will have to have a stem with more rise and shorter reach to fit you with drops. So if one of these bikes is comfy as it is with the flat bars, go shopping for different stems with the idea that bars will have to come upwards and towards you by a couple of inches.