The internal wiring from the front switch on the range going to one of the 8" heating elements has degraded and needs to be replaced. The wiring on the other 3 elements is fine. What type of wire is suggested, that is, what covering is required? Where can I purchase the wire and also heat resistant sleeving to dress the wires?

You buy the wire at an appliance parts store. It is hi-temp wire covered with asbestos or something similar. You will also need hi-temp terminals for each end that are chrome covered. Any other type will melt rapidly. I would think that you would use 12 or 10 gauge wire. It is usually on a rack along with different types of terminals in many parts stores I have visited. It's sold in a plastic package with a length of wire coiled up inside. You can also get it online here: #12 High Temperature - 250°C (482°F) TGGT APPLIANCE WIRE price per 10ft - Electrical Cables - Amazon.com for $11.20 for 10 feet and you can go here for hi-temp connectors (and they may have wire, too.): High-Temperature Temp Terms Terminals and Splices - Molex