There are a lot of alternatives to SIPs out there. Folks with different backgrounds will debate about whether the SIPs or the timber frame are the redundent "layer." You can build a SIP house without a timber frame inside it, or you can build a timber frame without SIPs around it, or you can use both. Search the forum for "built up" roofs. Also, some of the classic timber frame books have chapters about ways to build walls and roofs, without SIPs. You've got two years to research it. Good luck.