I just got some sourdough starter going with the Oregon Trail, and it seems to be doing quite well. However, the guide calls for using potato flakes or potato water to feed it- having neither, I hoped for the best and used some arrowroot powder instead. I'm not sure what the purpose of the potato stuff is- is it just food? I figured it might be primarily for providing starch, in which case the arrowroot should be fine.
I feed sourdough starter with strong white bread flour mixed with water to make a sloppy dough. This is a common method in the UK and it seems to work well.
I've never used anything other than good old bread flour or all-purpose flour. I've had a lot of success with Peter Reinhart's system of doubling the weight of your starter with equal parts flour and water. In other words, if your starter weighs 4 ounces, feed your starter with 2 ounces flour and 2 ounces water. Hope this helps!
I've only ever used white flour and water, nothing else. I mix mine with equal weights of both to get a 100% hydration starter.
Nothing else is at all necessary.