if you go custom build be prepared for months of researching, designing and building and it also begins to become expensive with all the parts and tools you'll need to buy.
i used propane (R290) for my first build because its cheap, no laws to dispose of it and readily available and can hit -40c @ 1bar of pressure. its can also work with both esther and mineral oil. my build ended up in the garage gathering dust, just didnt have the room to use it
MW