Small Magento development job

I have a Magento store. People can order custom items by the letter, which I then cut out. They choose the size and font adn currently enter their text in a text box and manually count how many letters they're ordering, which they then enter into an amount box. While this works, it's not ideal.

I would either like:
a) the amount of letters to be updated as they enter their text, or
b) they enter their text, press a "calculate" button and it gives them a total, which they can then add as an item to their cart.

I still need the text to go in the cart as a line item, so I know what to send them. It's just the amount field which needs to be auto-calculated.

This is Magento 1.7.0.2.

If you think you can do this, or have any questions, please let me know. There may be further additions later on, including rendering the text as a gif or png so they can see what it will look like.

Many thanks,
Simon