I'm waiting on about NZ $450 worth of 10mm thick M-foam and glass fibre from Gurit. I reckon it's enough to build 6 higher wind robust kiteboards. The foam is expensive until you buy it in bulk, then it's not so much. I wouldn't use EPS though, wrong characteristics and absorbs water too easily. I love Paulownia for boards and have made several but a proper marine grade foam combines impact strength with tensile strength at a good weight. It is less environmentally friendly but for those who think a wood cored board is an eco save, they are ignorant of all the other chemical processes that go into board manufacture. For a surf board, you will need composite fabrics to keep it curving right or pick a specially curved tree and make just the right cuts! Or laminate the board from sheets of Paulownia and glue it into the right curves.
Just get an eps blank and go for it, you will not regret the feeling of cruising on a sled you built yourself. The reward and satisfaction is immense and addictive. For inspiration, if you are on Facebook join Backyard Surfboard Shapers.... you will be amazed at what some of the new guys are producing and at the same time realize that it is totally possible create a great ride with your own hands.
Don't stress if you make mistakes.... chop it out/tear it off and do it again.