Well.....I'd first stick with stock injectors..putting larger flowing ones in without a tuner to adjust fuel tables will result in your engine running rich..which = poor FE and poor performance, so don't do that!
Next, I'd just send my injectors out for cleaning, they can check function of them and tell you if any are bad.
Fuel rails you can clean yourself with carb cleaner...I would be very surprised if they were at all dirty.
Before investing in an engine, do a compression check to see what you are working with.
My '01 4.7L is at 356,000 and while it burns some oil...mostly from worn valve guides...it still starts right up, and still has plenty of power. A worn out engine will be down on power.