Don't promise to visit her unless you really will. Write their address and your return address clearly with block letters. Be sure to include the country on both addresses because you're writing internationally. If you have them, include a few return address labels for her to use on letters back to you. Remember, our addresses are as foreign to them as theirs are to you.
Word processors have their pros and cons.
It's possible that she may think your letter is just one of many. However, because you can generate huge numbers of first letters of impeccable quality we feel they are a big advantage. If your handwriting is very bad we suggest you use a word processor. If your handwriting is very good and you have the time then pick up your pen. Use a nice pen and decent stationery.