Without knowing what part of the world you're in, it's a little hard to recommend a 3rd party.

There's a lot of material available. Perry wrote a Stairway that's available here. There's good info in Books Online, pluralsight courses, blogs, books. You can (and should) set up test VMs and try scenarios out.

I strongly recommend you don't make your first HA/DR design one that's for a mission-critical system of that size. You're doing your client a huge disservice if you tell them you can do it without experience. The best advice would be for them to get a specialist in.

Incorrectly designed/implemented HA/DR solutions can cost a business millions if there's a disaster and the implemented solution can't meet the SLAs. Companies have closed as a result.