Fabriano is my personal favorite when it comes what paper to use with watercolor. It's a high quality, sturdy and ideal for making custom personalized cards.
Fabriano provides both cold press and hot press paper types, and can range from 65lb to 300lb.
From experience, the best for a foldable custom card is 65lb, thicker paper can crack along the spine when folded, so a thinner stock is better if you're planning on that style. Of course you can always go post card style and not worry about cracks and folding!
After you've chosen your paper, we recommend taping down the edges. This can help with paper buckling, which can happen when too much water is applied to the paper. Additionally taping down the edges creates a border for your piece.
Next you can begin painting! If you'd like to try something fun you can add cinnamon to your watercolor to achieve a rust look. Salt and or sugar can also yield some fun results.