I'm planning on using HDPE as I picked that up from other build threads. For now I have ordered 4x 3.7V LiPo batteries and the motors are 2x Philips DC Geared Motor, Brushed, 12V DC from RS (330rpm). I worked out with some maths that would give the robot reasonably steady speed with 5cm radius wheels. For the first iteration I'm learning as I go so will decide on different motors if those don't work out. So far I've gone for cheap to mid range options.