Mike,
First, welcome to ExPo.
Well, not quite; I've only had my truck since 2003! But I know what you mean, large racks on Rovers are very common in the overlanding world. A few points from me, which do reiterate some of the above:
1. As said before, you want to put as little weight up high as possible. My 110 has a lot more up high than I like, but I accept that as a limitation to where I can go
2. Obviously you need a rack; roof tent, kayaks etc.
3. Your goal should be to keep the rack itself as light as you can while still maintaining the strength for what you need to carry. Think light materials; expanded metal flooring is very heavy. A lot of people use the plastic flooring from animal kennels, light and weather proof. Go with the lightest gauge metal you can get away with for the rack structure; the main weakness of the rack will be it flexing, rather than the weight you put on it.
4. Be careful with your mounting system. On the 110 pictured, I have a full cage that is mounted to the frame and doesn't touch the body anywhere. The rack is attached to that. Avoid splitting mounting between the frame and the body as this will just result in stressed and broken parts. The body does move significantly compared to the frame.
I've included a couple of pics that might give you some ideas. Good luck!
cheers