I would like to use FPD for selling metal lettering (for gravestones), so it's crucial for me to have 3 things:

1. price increase per character

2. live price change (woo or opencart)

3. own additional fonts

Is this possible?

Sorry, there is currently no way of charging per letter with text elements.

Unless you create what's called designs for every single character (you could assign them individual prices). However you need to create an image file of every letter you want to offer and add them as a "fancy design". A fancy Design is for example this:

http://fancyproductdesigner.com/product/t-shirt/