Dread Necromancer (Heroes of Horror, 85) 8 allows you to use Animate Dead for 4+Cha Mod/Class Level instead of 4/Caster Level.
That is the first thing that I thought as well, but you are talking about 6 spell caster levels before you get animate dead, and another 2 before you get the bump in the amount you can control. At ECL of 17, I think that will come a little late to be really useful. The dreadnecro levels will give some touch based adds ons, but I don't think it is worth it. Even if you advance Souleater out to level 9 I think (ECL 16), you will get a number of shadows under your control (when you e drain targets) which could be better depending on how many you can get at once. I am without my books, but even this might not be all that optimal despite getting the Full BAB and an extra drain per attack at level 7 from the soul eater.
Probably your best bet for armies is to take undead leadership. That gives you a number of undead minions to control (same as leadership, but everything is undead). You can still use the Dreadnecro as you cohort. He can always over produce (IE, make more than he can control so they go free willed) which you can use to explain how you replace lost minions up to your leadership chart max.
If you really want to go with spell casting levels, a dip into Ur-Priest followed by whatever divine PrC you are looking for could work, but it is very feat intensive (at least 2 feats for entry and then the temptation of divine metamagic). A less optimal, but less feat intensive alternative would be Divine Crusader from Complete divine. It gets spell progression like the Ur-Priest, but only casts spells off of one domain. I think eberron has the necromancy domain which would give you animate dead and other goodies. The only feat requirement for entry is weapon focus with your deities favored weapon (which you might be able to get from your fighter bonus feats if you can change what you had before). The class is also 3/4 BAB progression and does grant weapon specialization at level 5 I believe. The spell casting is off of Cha, so that would work well with leadership if you go that route.
There is also the bone knight in one of the books that everyone talks about. Not sure if it is as useful i f you are already undead