So you're wanting undead models (only zombies, or also skeletons and ghouls etc?), but they'll all count-as standard Bretonnians? But your Lords and Heroes (or at least the magic users) would mostly/all be 'alive' models?
I like the idea, so long as you can come up with some background fluff, e.g. they're a regiment from some city struck by a dark magical plague, and have sworn themselves to revenge even beyond death. Or whatever...

Obviously, you need to differentiate from the look of a Vampire Counts army. In particular: no vampire stuff, no bat wings, in fact nothing overtly 'evil' at all (unless you want Evil Bretonnians, but that sounds too similar to
VC).
Some model conversions seem pretty easy: men at arms, bowmen, yeomen. Knights would be a little harder, due to requiring zombie/skelly horses. Some modellers have had good results gouging holes in ordinary soldiers/horses then partly filling the holes with greenstuff bones and guts. Plus a few skeleton limbs here and there.
Others like pegasus knights would be quite difficult - other than tomb kings carrion, there's not a lot of undead feathery winged creatures