Hi Timey,
that is a great start, one thing i would recommend, when designing, is a lot of people will ask for sizing. and you will need to make sure if you size it will your pattern still sew together properly.
Testing the pattern is very important, before i used the rami style i attempted my own pattern, i had it test printed and it didn't work out, so i switched to the pattern everyone else used which was Rami, and after a couple of years i designed my own pattern and test printed and it works for sizing and pattern design.
as for using the Art work other have done. if you are looking at it for reference, and not tracing or copying textures then you are ok.
if you are tracing lines, placement i would not recommend to try and sell patterns or prints from it as it would be considered unethical.
another tip start small do not take on a huge amount of work, start small get a feel for the work load, time it takes to complete orders, you can get a bad reputation really quickly by not completing orders. always be open an honest with potential clients or time frames and costs. price according to what you thing your time and effort is worth not the Market.
This needs to be fun for you, if it becomes a burden it will turn into resentment it will affect your work as well.
hope some of this helps and if you have any other questions send me a message. happy designing !