bmi scales would be useless for your purpose and besides you can just find bmi calculators for free on the web... a bmi calculator only measures your weight per some unit of measurement (can't remember which one) so if you are very muscular with little fat it could show you as obese and if you're very fat with very little muscle mass it could mislead you into thinking you're in the healthy range or even underweight...
I have a body fat scale and I dont think it is accurate but when I have done skin calipers I have gotten way different results. Calipers are the way to go in opinion.