Check the manufactures specs for using PVC. For example: You can use PVC venting on the Rheem Prestige line. However Rinnai's Ulta series you can not use PVC.
Since your furnace uses PVC for venting, I am about 99.9% certain you can NOT common vent with your furnace and a condensing water heater. Both have a positive pressure vent. One of the pros will chime into verify this.
What's the temperature of the incoming water? You will want to look at the temperature rise charts to select the right size model to fit your needs.