Buckskin 1................you are at a good place here on this forum to learn. You could probably take a week of reading through all the topics, and still not get it all done.
Here's my take on your question and situation. Book/dvd/video learning resources are the Stohlman books, J Watt dvds, Cheney videos, Gomer videos all good for the beginner......especially the books and Watt dvds. Harwood's dvd will be of much more benefit to you after you have built a few.
As for supplies and tools............I am going to disagree with some suggestions previously given. For trees..........buy the best you can afford, and certainly stay away from the ralide stuff. If you build a decent saddle on those trees, then most likely you will have a saddle to sell that is worth less than the material cost in it. The good quality mid range trees from Timberline, as one example will only add $125 approx cost to the total saddle, but provide much more than that in quality and value. As for the leather, you could economise there a little maybe, but..........I don't know what a side of Tandy's skirting leather cost, but you can buy #1 Herman Oak 13/15oz skirting from Panhandle Leather for $156 per side, plus shipping. As for tools...........unless you plan to build only one saddle, and then never use the tools again for another saddle or leather project..................buying cheap inferior tools is very much false economy.
Just my opinions, and I intend no disrespect to you or those who offered differing suggestions . Best of luck to you on your new saddle making journey. JW