The Emperor Titan is 4000 points of pure kick ass. What you need to go over are the rules pertaining to Super Heavies in general, and not just the model itself and the weapons it carries. (ie - Structure points, Void Shields, multiple targets)
In a nutshell, the Emperor Titan has a lot of Structure Points and a lot of Void Shields. To scale, the model stands about 4 feet tall and is about 3 to 3.5 feet wide. If you check out some of the pics from the Megabattle, there were 2 that were made for that day.
You can't buy the model anywhere (yet), but if you feel creative enough to try and make one, the sky's the limit as long as you have the initiative to make it.
As far as which Super Heavies you can bring, technically, you can bring anything that is part of the greater Imperium. So Hellhammers, Baneblades, etc, are all legal for Space Marines. Fluffwise, I don't know how that mixes with Blood Angels, but I'm pretty sure it wouldn't be a problem. Ask some of the people in the store who know the storyline behind 40K better to see if it matches well with them.