The O rings for the injectors came with the kit I got from Rover Connection but it never came with the upper inlet manifold gasket, had to re-use that one, or the gaskets for the heater and coolant pipes that plumb into the inlet manifold. It appears the AB one is better and has everything. As the injectors sit in all that heat I would replace the O-rings as a matter of course. As far as the timing chain goes if you are just doing heads then there is no need to do anything with the position of the timing gear. As far as removing the rocker assemblies make sure you mark left and right and back and front. I used small cable ties around the shaft, one for right and two for left, both at the front end. That way you can give them a soak to get all the crap out. Use a cardboard box to put the rods in and mark so they go back in the right position. Take lots of photos so that everything goes back the same. I put small luggage labels on the pipes and connectors but they do get wet and so can smudge. Every bit that came off went into a ziploc bag and I wrote each step down to make re-assembly easier. It helps to make hand diagrams too. Don't bother re-using those crappy LR hose clamps change them for proper screw hose clamps. Make sure to get all the crud out of the bolt holes and run a blind tap down them too. Cotton buds with brake cleaner on works in the holes. I put the exhaust manifolds on and then put the exhaust pipe on and it was no problem. If you don't have a dial gauge for the bolt heads then what I did was torque per manual to the correct setting, I believe 15lbs then using a white paint pen draw a line from the middle of the bolt head to the edge. I did it pointing to the outside edge of the block and then do the two 90 degree torques. You will need a long breaker bar for this and it is scary the first time as you think the threads will strip. You will have to push a bit of the bulkhead in by the drivers side lower head bolt to get a square approach to it.
As long as you go as you have been then it will be o.k.
It pays to take your time.
Godd luck
Bill B