I have a 100 yard range right now, as well as a nice wide pistol range, and am planning a 300 yard range once I get ambitious enough this summer. I don't really have any advice for you though since my property (20 acres out on the edge of town) is surrounded by hills, and berms.
One thing I'm looking at for my 300 yd lane, is using old tires, in two alternating rows, filled with sand, for spatter control and a little extra protection, in this one area.
One piece of advice I've found works pretty well to keep the weed down is tarps. In my pistol range, I layed out old tarps and the weeds can't get through it obviously. It also makes it super easy to pick up brass, and a leaf blower at the beginning of the day keeps it looking nice.
Just gotta keep It layed down flat so you don't trip on it. But in your case... I think it might work out great!