1. The VS tanks are very good tanks. I have four of them, and have yet to encounter any problems with any of them. *knock on wood*
2. You do not need to buy different tanks (ex. Sherman vs Tiger). Two Tigers can battle each other, just makes sure that the tanks you buy have different frequencies.
3. They use standard 27mhz crystals although I'm not sure if you can use aftermarket crystals (I've only used the VS tank crystals)
4. All the different models are fairly equal. Although I did have a little problem with my Tiger occasionally throwing off a track when I do a hard turn on uneven surface.
5. There's not much selection in the 1/24th scale. The only other 1/24 scale RC IR-capable tanks I know are the Forces of Valor tanks. STAY AWAY FROM HENG LONG 1/24 TANKS.