migration to 2000

i have an nt4 platform /pdc and bdc with exchange 5.5 and a proxy. users are aprox 200. migrating to 2000 sever with active directory seems to be my focus for transition..I would like to know if migrating all the permissions, users and settings is abest bet or to start from scratch with a full install instead and reconfigure all previous info.. i ideally want to have to identical severs for complete redundancy any comments?