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I am normally a 40k player, but a nearby store is doing a grow league for WHFB from 500-1500 to try and get some more people into it.
I split a box of IoB with a friend and took the skaven side, as I always wanted to do a skaven army.

As it stands, my 500 points list is:
Engineer with doomrocket
Cheiftan with BSB and enchanted shield

28 clan rats with shields and musician
31 slaves with a musician

2 rat ogres with a packmaster

1 Warp Lightning Cannon

From what I can tell this should be a decent list at 500, with plenty of wounds and some hard hitting weapons, but I am unsure as to how to take this list up to 1000 (I also want to keep it fairly cheap).

Make the engineer a level 1 or 2
Add another chieftain.
Add 2 or 3 rat ogres
And 30 more clan rats with a mortar.
Or more slaves and add the mortar to the clan rats you already have.

Should be effective enough at 1000 and easy on the wallet

So I think I am going with this list:
at 500

Chieftan bsb, enchanted shield
Warlock engineer, doom rocket
30 skavenslaves, musician
31 clanrats, musician, standard, poison wind mortar
Warp lightning cannon
for 498.

At 1000:
Greyseeer

Chieftan bsb, enchanted shield, biting blade, ironcurse icon

skavenslaves x40 musicians
skavenslaves x40 musicians
clanrats x40, full command, poison wind mortar

doomwheel
wlc

at 1.5k:
Greyseer, d scroll, warp energy condenser, ironcurse icon

chieftan bsb, enchanted shield
plague priest, plague furnace
warlock engineer, doom rocket

skavenslaves, 40x musician
skavenslaves, 40x musician
clanrats 40x shields, full command, poison wind mortar

Plague monks x20, full command

doomwheel
wlc

Couple comments:

1. BSB is probably the most important model you'll have on the field. Moreso than the general most of the time. Kitting him out for defense instead of offense is usually best, and the typical build is Charmed Shield + Talisman of Preservation. Gives him a 4+ armor / 4++ ward.

For 500 point games your build works well though.

2. Consider a warlord instead of a grey seer at 1,000 points. Your general gives up an extra 100 VP if he dies, and a grey seer with no defense whatsoever will die pretty quickly I'm afraid. While a warlord isn't as killy as other armies' lord choices, he can hold his own against most other infantry. He's also far cheaper than a Grey Seer, so you have some room for items and such. For your magical oomph, a couple engineers will do fine.

3. Your 1.5k list, 20 Plague Monks won't last long, and I think you'll find your Plague Furnace is stuck. Might want to either buff the number of monks or leave it for 2,000 points.

4. A second WLC is never a bad choice.

Good luck!