Actually Jersey’s speed limit is 40mph, whilst Guernsey is 35mph, but lots of 20mph zones.
Channel Islands cars will have had a lot of stop/start driving, and brakes get a good working over, most Journeys will be about 15 minutes fir Guernsey, a bit longer for Jersey.
Most of the rental fleets get changed every year and will generally have low mileage. Check for pavement wheel damage, and also check the wheel alignment, they will probably also be on their 3rd or 4th door mirror.
As for salt damage nothing from when it snows as this roads don’t get salted, but a windy high tide will see many being driven through foot deep salt water.