The first place to start is to run a cranking compression and or leak down compression test on your engine. That will tell you if the cylinders are sealing a way that they can safely accomodate some performance additions. Does the car smoke blue or burn significant oil between changes? If so, modifying it for more power will probably only exacerbate those problems. However, there are many 5.0's out there with more original miles than yours and much more significant mods that are working just fine. So if the engine's been reasonably cared for, you shouldn't have any problems. Email tmoss and/or Thumper about the changes you're considering - they have good information about what you can expect in terms of performance, and what the impact of using the stock throttle body and maf will be. You shouldn't have to make any fuel injection changes to accomodate the pieces you're thinking about, although as suggested above, the larger throttle body and maf changes may help out a bit. Ask tmoss - he'll know.
The stock rocker arm is a lever. When the pushrod pushes up on one end, the other end of the rocker pushes down opening the valve. It's not an equal length lever - it's offset; it is 1.6 times longer from the fulcrum to the valve than it is from the fulcrum to the pushrod. So, if the cam lobe is .300" tall, when you push the rocker up .300", it pushes the valve down 1.6X.300" = .480". When you substitute 1.7 rockers, they have more leverage than the 1.6's. Using the same example, if the cam lobe pushes the rocker up .300", then the 1.7 rocker pushes down on the valve 1.7X.300" = .510". So, with the higher ratio rocker you get more peak lift at the valve, and the valves are open to higher levels a bit quicker than they would be with 1.6 ratio rockers. It has the effect of putting in a camshaft with a just a bit more lift than whatever you have in the engine. Because of that, it stresses the valve spring a bit more. But, if you're going with Thumper heads - just tell him you're gonna run the stock cam and 1.7's and he'll pick valve springs that will deal with that just fine. Oh (edit) - I wouldn't expect significant gains from the rocker change. Your big hits will be the heads and lower intake - so if there's one of the three that you decide you can't do - it's definitely the rockers.