The stamped steel sensors on Ebay are the cheapest. You might need to spread the load over multiple sensors to get the 100Kg load bearing capability.
With a suitable enclosure there's no need for the load cell to be exposed to the weather. High humidity and temperature gradients are your main concerns.
Apply a liberal coating of bituminous paint then re-calibrate over the required temperature range.
Compensate for long term drift by setting up a duplicate system that you can fit with a series of known weights.
30g resolution outdoors it going to be tricky as the wind can easily apply that sort of positive/negative lift force depending on the shape of the thing you are weighing.