About three miles from me is a big plant farm and every fall they get tons of sawdust from a wood mill. They simply use the sawdust as a starting medium with no extra stuff added. After they have the starting beds prepared with the sawdust they sterilize the beds with steam. Then they plant the seeds. They have been doing this for many, many years with great success.
I do not start seeds indoors any more but I have used this method in the past and it works fine.