Heavy weapons only appear in:
command sections (dakka-esque weapons like heavy bolters, dual stubbers, autocannon)
fire support squads (as above)
anti-tank squads (lascannons only)
Mortar squads (take a guess)
Is there a points limit on the Macharius? The Baneblade cant be used in anything under 2k points, what is the limit on the Macharius if it has a limit?.
There isn't a points limit on either. The rule is that super-heavy tanks can only be used in a seperate detachment of 1-3 vehicles.
In the case of the Death Korps, thats 1-3 Baneblades, 1-3 Macharius or 1-3 Gorgons.
This effectively counts as a force organisation chart by themselves, which means that you can only use them where your opponent will let you use a second force organisation chart (which is merely 'suggested' as being over 2000 points) or where they will let you use a detachment of super-heavies as your starting force org chart.
Basically, they're completely 'as agreed with opponent'.