Looking great so far. I have always had an interest in buildng another one of these Maus kits. Yours is looking good form the get-go.
If your looking for aftermarket for the Maus, there are a few good items to have. Al LaFleche already mentioned the Jordi Rubio 12.8cm barrel (JR-24). You may want a 7.5cm Pak 39 (JR-27), for the coaxial gun (it works as a good substitute) as well.
Eduard has a PE set for the Maus too. Eduard also has an aluminum barrel for the 12.8cm Pak 44, but its the Jagdtiger late barrel. I'm not sure this barrel will fit the Dragon Maus, as the barrel is machined for the Dragon/Revell Jagdtiger kits.
Verlinden makes a nice resin Engine insert for the Maus. This is worth getting, and will save you alot of "blanking out and painting black", and will give your Maus kit an extra boost.
Remember, the real Maus was sent into battle partly uncompleted. Only 2 saw action, and one was blown up by its crew. 1 was captured by the Russians, and is at the Kubinka Museum today.
A good source of reference for the Maus (and inexpensive) is the soft-cover book by Thomas Jentz "Panzerkampfwagen Maus", and its by Darlington Productions.
Good luck on your Maus kit, and keep sending us more pictures !
Cheers !!
Robert