memories
One of my favorite characters was a rainforest indian themed cleric. He used a spear and a wood/hide shield was his only 'armor'. I never thought of using the longspear because he liked to throw his spear (it caused blindness).
Benefit of a longspear over a shortspear is one die bigger on damage and x3 instead of x2 crit multiplier. reach has pros and cons imho.
I'd rule two feats. One to allow you to weild a shield larger than a buckler with a weapon, see buckler below. (this feat would probably negate AB penalty for buckler, and light would be -2, heavy -4, no towers) Second feat to grant shield AC bonus while using the weapon in that hand, requires first feat and BAB over +5, something like that.
SRD
Buckler
This small metal shield is worn strapped to your forearm....You can also use your shield arm to wield a weapon (whether you are using an off-hand weapon or using your off hand to help wield a two-handed weapon), but you take a -1 penalty on attack rolls while doing so. This penalty stacks with those that may apply for fighting with your off hand and for fighting with two weapons. In any case, if you use a weapon in your off hand, you don’t get the buckler’s AC bonus for the rest of the round.
You can’t bash someone with a buckler.