The 1/48th-scale AFV Club Sturmtiger Michi posted looks like a good kit (I haven't built one, but saw the parts once IRL, and they looked very good). I've got their German halftrack kit (unbuilt), and that is marvellous with tons of detail and lots and lots of extra parts.
As for the Mörser Karl (self-propelled), Trumpeter make both the actual gun vehicle and the PzIV-based ammunition carrier in 1/35th scale. I remember reading reviews of this and the Dragon model, and the Trumpeter kit usually was described as the better choice. Cost is ~60 Euros for the Mörser Karl, ~40 Euros for the ammo carrier, depending on where you get it.
The Mörser Karl, however, is a very complex kit - the railway carriage version allegedly has more than 1000 parts. I *think* I saw the kit at my local Modelzone store, but might be mistaking it for another of Trumpeter's large box kits (their E-100 is particularly tempting to me).