A chair is a high liability item, and the impact of the 300 plus lbs. person slamming into the chair from a standing position will require that dimensional stability and tensile strength be balanced against probable failure areas that would come from flow, stress, weathering and everything involved.
Assuming “good geometry” I would start with a filled PP to stiffen the part. I lived through this exercise as a designer for Rubbermaid. We sampled various materials and fillers and compared the parts/product with our predefined performance (static load, impact, temperature, etc.) finish and cost parameters. I would suggest meeting with your material supplier or suppliers and sampling a minimum of 3 materials.
The most critical element is to have a good design first with special considerations to the structural stiffening members. Then use glass fiber reinforced PP, but remember the injection molding of glass filled PP is different from the injection of unfilled PP even if you are molding the same shape. The stiffness and the strength of the part depend on the fiber orientation pattern throughout the molded part. Such pattern is determined by the location and the type of the injection gate. The fiber length distribution which determines the quality of the compounded materials has huge effect on the creep modulus and hence on the long term strength of the par during application. It is not enough to use glass fiber filled PP without specifying the average fiber length. It is also important to consider the effect of the fiber orientation pattern and the fiber length on the warpage and shrinkage of the molded part which is completely different from those of the unfilled PP molded part.
PP is prone to surface chalking and a proper UV / AO stabilization package need to be part of the system as the suggested UV absorber and primary antioxidant will not be able to protect the surface of a PP based application.
I always recommend installing a close relationship and communication with your resin, master batch- and compounder supplier in order to get the holistic solution and not a fragmented one from different angles. There need to be a focus on potential risks and total cost and not price per kilo in these discussions as it can be very expensive in buying cheap raw materials.