I'm the proud owner of a Seacam 5D housing, which I think is really the most sophisticated to fit in your hand. The house I bought for 2300€ about 8 Months ago (2nd Hand). A couple of days ago I saw one 2nd Hand in the Web for 1999€, so 3000usd I guess is much, especially as the 5D MarkII came out recently.
I have the 100mm, 50mm and the 16-35mm (1st Edition). I consider this to be an adequate range. To start I would suggest the 50mm to be the most versatile, and thus the one to start with.
The 16-35 does deserve the Superdome (or equal from other Brands) best. This because of half-half and optical quality at its corner.
The 100mm is super for small animals, but is slow at focus, thus not for use in decent currents.
Personally I guess the MarkII does not yet add enough value to switch, as I consider my experience as Photographerstill to be the bottleneck
Hope this will help