Welcome to the forum.
You can build a very simple mill, if you have the skills. Adding things later is good, as long as you build with those things in mind in the beginning.
IF you go to the Linn Lumber website, they sell plans for $50.00 and also parts to construct a mill. I think this will be your best bet to at least look and decide where to go from there. With all the photos on their site, you will see how mills are basically built and you can scrounge or trade for what you need.
I have bought and am still buying cast wheels and shipping them down here with other machinery I have, in Fl.
I am about to start building a multi head bandmill, using motorcycle wheels. We have a small tree farm and will be sawing 14" and under logs, as we thin the stand.
If you have any questions or need assistance, start a new thread in the bandmill section,and we can all advise you.
I AM assuming, you want a bandmill, correct ?