This is a useful tool for making army lists:
http://wm-selector.appspot.com/army_selector.html?list=chaosChaos Starter is:
2 Chaos Warrior units
2 Chaos Marauder units
1 Chaos Knight unit
1 Chaos Hounds unit
+ 1 General figure
+ 1 of your spawn units makes about 900 points. You need 2000 points to use the second unit.
To answer your questions:
Chaos armies should always max out Chaos Hounds, as they are the sole "cheap" unit they can get. That means 2 for every full 1000 points.
Chaos Marauder Cavalry are much cheaper than Knights and are a good investment, as are Chaos Chariots. Chariots are only 5 points more and get the bonuse of extra attacks when charging in the open.
Chaos Knights are great but very expensive, almost double the cost of a "normal" knight unit in WM which is 110 points (Silver helms, Empire Knights, Kislev Lancers etc.)
Chaos Dragon Ogres look great but are too expensive to really use in a beginner list so I use the ones I have painted as Chaos Marauder Cavalry proxies.
The Chaos Spawn you have rely on being brigaded with infantry to offer something nasty and a shooting attack. If you're going to use them you might want to have a larger infantry block. Marauder Infantry are equivalent to High Elf Spearmen in stats. Warriors of Chaos are wonderfully tough but 150 points makes them a very pricey option that you can't afford to have out in the open.
A single harpy unit is worth getting as a threat to attack the rear of artillery or immobile infantry blocks.
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As for unit availability, that's a bit of a problem. Chaos, as Aquahog says, is one of the 6 "starting" armies from about the beginning of Warmaster in 1999-2000. As such it is more common than some others. That being said, Chaos Chariots seem to be rather rare. Chaos Hounds and Chaos Marauder Horsemen are never present in huge amounts, I think because these units were not in the starter box that you have.
If you post a topic in the trade forum here, you might occasionally snare a unit or part of an army to expand your own!
If you can wait, there are some Chaos proxies coming out of this kickstarter:
https://www.indiegogo.com/projects/evil-men-fantasy-10mm-miniatures#/updatesHe says he'll set up a store in the run up to Christmas.
If you need Trolls, the excellent Polar Fox webstore has suitable proxies that are very good:
http://www.polarfox.info/Taiga_Trolls/p2627799_12488305.aspx---
Dark Elves go for a high price still - but High Elves are Tomb Kings are among the cheaper armies to get on eBay or through trading.