No, they do not give discounts for buying more than one device.
And, the iPad Pro is great for drawing, but I would recommend that you go into a store and actually use one - try it out. They are wonderful, but they are not computers, so you best check to see if it will be suitable for whatever else you need to do. I would not give up my computer, but I also have an iPad which I use for art, reading books, and streaming movies to the big screen TV.
Oh, and the trade in program for computers is administered by third party companies and their final offer may not be the same as the initial estimate. Take photos of your device's condition before mailing it in just in case there is a problem with shipping. You are better off selling it yourself.