Hallo Ferry and Elisa!
I dit that bordercrossing 2 years ago, but i went from Guatemala to Mexico via Bethel. The road in Guatemala is full with big holes, but no problem. In Mexico the road up to Palenque is paved. The rivercrossing is easy, if you take the boat from the Mexican side just across the river it will be cheaper but you will need help from the locals to get up the steep hill to the road. I payed 35,- Pesos (Mexican) for my African Twin and me. If you take the short rivercrossing you drive down the road about 5 or 7km, than you hit the other road, take a right back to the river and get your immigration done in Guatemala. Don't worry about the boats, they are quiet big and they will help you to lift the bike inside!
Have fun, Albert!