Current Tamiya models all have metal tubs, or "lowers" as you mention. 

They all come with plastic tracks. 

As to which one to get, start with the one you like best..  If you want a shelf queen or to run about in the back yard, you'll find they're all good.  If you plan on battling, stay away from the KT until you've built a few others, as this tank takes considerable experience to keep running under harsh conditions (due to the wide tracks).

Sounds are pretty good on all of them.  The new KV and PZ4 have (imho) the best sounds.