Hi Scott
What you are looking for is a drum sequencer, and the best way to find out what’s available is to just type drum sequencer into Google, however be aware that most are designed so as to allow you to make up your own rhythms, rather then supplying ready made patterns.
The project you are considering has a steep learning curve and many pitfalls; therefore I would suggest you start off with OMB, which although very basic will give you a good grounding on how to put together a simple setup.
Multiple keyboards sound good initially, but due to the spacing required can be awkward to play together. (An organ has the keyboards in close proximity thus making them easy to use)
Don’t forget also that Wersi OAS instruments come in all forms, (The Abacus being the most flexible) and all the setup work been done for you, this means that all you will need to do is select what software instruments you wish to install. (Although when you see what the OAS software is capable of on its own, you will most likely find that you will not have to add many software instruments or samples to get what you want) The downside is the initial cost, however as you never need to change the instrument, (You upgrade the software) in the long term it becomes a very cheap option.
Hope this helps

Bill