Found an idea diff for the buggy! But... It's $900

It was on this site which is really cool and I'm sure I will be ordering some other stuff from them!

I have decided to start designing a small buggy to carry stuff up the stairs. I am going to slowly flesh out this brief properly as I come up with more ideas.

Design Brief:

Basic Requirements:

• Must carry ~40kg and it’s own weight up standard incline stairs.
  
• Must have facilities to hang shopping bags on or a suitable 'tray and cage' arrangement to carry and contain a load of shopping bags.
• Will be propelled and steered electrically.
• Will be operated remotely.
• Buggy will carry a web server and webcam.
• Buggy must be able to navigate standard 800mm minimum clearance hallways.
• Must have a balance mechanism to counter-balance load while climbing stairs.
• Buggy will be reasonably quiet.
• 4 wheel drive (diffs) - probably not a centre diff.
• 4 wheel steer
• Independant Suspension
  

Control System:

• Lappy.
• 802.11b wireless.
• 2 or 4 wheel steer modes.
• Controller(s)
• PWM for traction motor(s).
• Stepper Control for steering Stepper(s).

Telemetry:

• Per Wheel RPM.
• Per Motor Current.
• Current Draw of traction motor(s).
• Roll/Incline sensor.
• Slip indicator.
• Front and rear looking camera.
  

Interface:

• Visual Basic.
• Display of Telemetry.
• Allow Control of Buggy.
• On-Screen and Physical Controller.
• Fail-Over Controller that plugs into buggy.
• Front and rear looking camera.

Vehicle:

• Propulsion
• Traction Motor(s).
• Probably a VW generator (doubled ended).
• Components.
• Modular.
• Easy to construct using available equipment and materials.
• Drive Train.
• Wheels.
• Square form pneumatic wheelbarrow wheels with keys or splines on axels (not roller bearings obviously.)
  
• Power transfer
• Front and rear quasi diff.
• A bevel gear set from an angle grinder.
  
• Universal Joints.
  
• Steering.
• Arms actuating wheel hub.
  
• Stepper actuated.
• Both Software and Manual adjustment.
  
• Suspension.
• Arms.
• Springs.
• Towers?
  
• Brakes (Possible)
  
• Motor + lever on disc?
• Pneumatic?
• Hydraulic?
• Manual Handbrake?
  

Power:

• Batteries
  
• SLA Battery
• Bank of smaller SLA Batteries
  
• 24 volt series/parallel arrangement
• Charging
• AC Inlet/Built In Charger?
• External Charger?
• Careing for battery
• Auto discharging etc?
• Zapping.
  

Electronics:

• Circuit board Dress / Wire Loom Dress
• Modular design
• Plug and Socket interfaces for key components.
  
• Documentation of circuits!