I typically ran about the cheapest slann I could in the old rules, and just cast 5 first level pew spells with plus to cast, it was surprisingly hard to dispel them all so it was about as strong on firepower as two blocks of crossbowmen. The main reason I personally take the slann nearly every time is twofold... the scar vet can take most lords, and because the slann is pretty much the best general ever. The temple guard being stubborn and ITPsych re-rolling on three dice is plain stupid. It's practically worse then flagelants. The statistical chance of failing on LD 9 is significantly lower then 8, and the boost from 7 to 10 is even better... Whenever a group of skinks rallies or doesn't flee because of the Slann some of my opponents have begun going "wom wom wom wom...all glory to the hypno toad" Having a bsb without losing all your magic items and being stuck with just a hand weapon is amazing... I hate having a saurus bsb because then he becomes worse in combat. The other thing is, the toad is a near irresistible magnet for the worse crap my enemy has. I can't explain it, but for some reason just seeing the toad march up the center of the field just infuriates nearly every opponent I have to throw their absolutely most horrifying unit at it, blood knights, chaos knights, a bloodthirster, the common concensus is that toad must die.
On a similar note, in a 3000 points game I made the mistake of taking two slann in the same TG unit, and will not likely repeat that mistake, for your second lord, take the carnosaur, or take an ungodly tanky Old blood, just from a metagame perspective few people have enough models to play horde tactics in 3000+ and so tend to fill the points by point sinking into some hero, don't know how many times I've heard, hey, gotta fill points, mind if I borrow that dragon and play it as a.... "insert stupidly powerful lord". Me: "err... sure".
On a personal note, most lords are terribly overpriced and won't return their points. The oldblood is especially un needed since the scar vet already has max powa. If you really feel cheap, take a slann bsb with the diadem of power, the battle standard,and the Becalming cogitation. 350 points of great leader that can cast one or two critical spells, and will still hose most crazy magic lists just by becalming their lord.
I prefer an Oldblood but like you stated i always take 2 lvl1 priests. One with 2 dispel scrolls and one with the Cube of Darkness. Keep those saurus units screened and hit hard. They should stand up to all except for any crazy tooled up chaos warriors and demon bloodletters.
