To answer you questions:
1) for wheels I have just started using wheels salvaged from in-line skates. These are 78mm to 72mm diameter and made of semi-soft polyurethane. They are smooth with no tread but stick good to most surfaces. Buy the skates used at the Goodwill store for $10 and get not only 8 good wheels but 16 good sets of 608 size ball bearings.
2) other option is wheels for 1/10th scale off off or on road R/C model racing. These attach using 12mm hex drive. Some on eBay are very cheap the "Wild Thumper" uses these 12mm hex drive wheels. There are hundreds of different kinds. Look at the place who sell R/C cars Just find bolts with 12mm hex heads and drill them out for motor shaft and screws.
3) suspension: Automotive suspension is designed for driving fast over mostly smooth roads. It's main job is to absorb impacts and damp oscillations. It is NOT well suited to robots that drive slow over rough terrain. Notice the Mars rovers that NASA landed on mars look nothing like a four wheel drive truck. They use rocker bogy suspension which is ideal for going slow on very rough terrain. Note rocker bogy would not work at all on the family car, the ride and cornering would be horrible. Even a good "rock crawler" truck can not climb over rocks that are taller than the weeks but a rocker bogey can roll over a rock that is twice the wheel diameter. The R/C modelers are making models. They use automotive style suspension because they want their model to look like the real thing. And they are not building robots, they are building R/C cars that have a human operator
4) if you really care about performance you have to do your engineering homework. You start with a written set of requirements. write down the type of terrain height of obstacles and the required speed and the payload size. From there you can work out wheel size, torque requirements and the suspensions type. Build a cheap unpowered wood prototype to verify your design.
5) Metal parts look nice but there are other "soft" materials that may work better. The plastic from cutting boards is very good, it's tough and has a little "give". I think "soft mechanical structures" are the next big thing in robots. l biological systems are soft, even bones and joints.