What sort of point total is most common for you? If you are looking at 2k+ (2500 is most prevalent for me), I think a Slann and some Temple Guard would be an excellent investment. If you really need to save money, you can convert/proxy some Saurus Warriors and just buy 1 box of Temple Guard (there's enough extra Halberd arms to make 6 Saurus Warriors into Temple Guard I think... it won't be the prettiest, but it will work and save money), with 2 boxes of Saurus, a Slann and maybe a Kroxigor to make a competitve list including what you already have that way.
Unfortunately, Warhammer isn't a hobby that lends itself well to playing on a budget. There are also websites you could try to find some older/unwanted stuff for cheap.
While it's not an ideal list, the following could work with those purchases at 2k:
Slann- BSB (St. of Discipline), Cupped Hands, Bane Head, Obsidian Trinket, 3 Disciplines (I like Becalming Cogitation, Focus of Mystery and The Focused Rumination)
Skink Priest- lv1, Dispel Scroll
Skink Chief- Light Armor, Shield, Stegadon War-Spear on an Ancient Stegadon
26 Saurus Warriors- Full Command
23 Skinks- Musician, Standard +1 Kroxigor
16 Temple Guard- Full Command, Banner of Eternal Flame
6 Chameleon Skinks
Salamander & Handlers- Extra Skink
Salamander & Handlers- Extra Skink
The Temple Guard unit is a little small and it's heavy in points for Lord and Hero choices at 2k, but provided there aren't any cannons to take out the Stegadon, it can be competitive if played well.
