Replacing the lifters is not an easy job; I've done it. It's an all weekend job for a hardcore driveway mechanic.

I don't know that you can rely on the oil pressure measurement as a good indicator of the oil pickup tube o-ring seal. It only take a little stream of bubbles in the oil stream to make noise in the lifters.

I've done the oil pickup tube replacement also, and it's not real easy. It's a one day job for the hardcore driveway mechanic. The parts are real cheap.

The price you cited on the lifter service kit is WAY high. I can tell you a dealership where you can order the kit for ~$170 and this is the Volvo kit.

Keep in mind that you want to have a good mechanic who has the right tools to install that top head section and one who will do a goo djob in dealing with the chemical gasket do the job. You don't want to be finding yourself in the situation of the poster in this board who has a cracked top head section which almost certainly resulted from a 'not so good' mechanic.

I did the oil pickup tube o-ring first, then the lifter kit. Because of the cost difference in the jobs, I would reccomend doing them in that order, but you'll probably end up doing both to fix it just as I did.