The external chambers are for a net performance benefit. Basically they just allow for a larger volume of the viscous fluid which in turn allows for better heat dissipation. Heat build up is one of the major causes of shock failure.

Gas pressurized shocks just have pressurized gas above the fluid but does not necessitate the external chamber.

As long as you get a shock that is rated properly for the application and it has enough travel to accomodate full articulation of the suspension, you'll be OK.

It helps to understand how a shock absorber works when deciding which ones to buy. Basically, you are trying to move a large amount of fluid through a very small hole. The harder you try to force the fluid through the hole, the more the shock absorber resists that movement. It does not hamper total suspension travel, only how fast the suspension can articulate.

Wikipedia has a decent write up on it:

http://en.wikipedia.org/wiki/Shock_absorber