Personally, I'd buy from EngineBuilder. He posts here regularly, is very helpfull, and one of the most affordable suppliers around. As such, I tend to do business with folks like that, and recommend them to others.

The kits themselves all seem to be the same. You're not getting something special from any of them apparently.

Installing the timing cover without removing the head is not particularly hard and can be done quite successfully.

Length of time to failure for the oem plastic guides varies tremendously. One of my 22RE engines has about 330,000 on the original guides and chain.