I know very little about materials but as a mountain biker, I know that bicycle frames can be both strong and light, and as a maker, I know that mild steel square tube is readily available and easy to weld.
Initial research shows that steel is heavy, yet strong, and aluminium tube is also readily available, is lighter than steel, but not is not as strong. Thinking about the loads and forces that this wheelchair is going to experience, it needs to be strong. In order to keep the battery weight down though, the frame needs to be kept light (so that the batteries need provide less power). The material that bicycle frames are made of seems like the ideal solution. A quick Google search says they are made of alloy, composite material of aluminium and some other element.
Interestingly, I haven’t yet been able to find alloy square tube in the shops, however… carbon fibre seems surprisingly available and affordable!
More research required…