Having been a position of having a limited budget, and buying a horse with mild navicular because his condition made him affordable, my advice is DON'T DO IT. I broke my heart and my wallet over that guy, he was the most awesome horse but we constantly struggled with shoeing, drugs, rest and exersise to keep him sound.
Even when he was sound I was worried about him being lame
As others have said you CAN manage the condition, and she may well do a great job for you for years to come, but equally well you may end up with an uber fancy lawn mower. The uncertaintity of it would be enough for me to say pass