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there are only a few things that cause low oil pressure:
-bad oil pump
-plugged screen on pick up tube (lots of people have this problem, I did)
-no o-ring on end of tube
-bad bearings (hard option because of the crank girdle wound need motor on stand) you can grab ahold of crank and rods and shake and pry (don't be a gorilla!) if they move too much, could be a problem.
-general trash in motor (change oil in more frequent intervals and cut oil filter open to look if its jammed up with trash)

 

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you can get the oil pan off easy (well after holding your tongue right). on a 2x4 a 4x4 you have to lift the motor.
I would do that, you can pull the tube off and check and check for blockages and o-ring issues. This will cost you around $70 I + oil. I would do that first. The oil pump is hard to get to (behind timing) and would be another $200 to do. So try the cheapest and easiest fix first.

To do the bearings, you need to pull the engine, these motors don't have main caps like a chevy 350 or the older 4.7l they have a girdle witch is most of the bottom of the engine.

ps if you have to do bearings remember they are matched in each journal and are not the same sizes. the will have letters on the HO cranks.

 

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when I rebuilt mine it cost around 1500 with the machine shop and the parts. I then spent another 850 on heads because mine were not able to be rebuilt. So around $2500 with odds and ends. You can build it for cheaper, but never buy the timing kits on ebay! or if do get good tentioners and cam sprockets....

I've seen them on craigslist for 1600$ with warranties. I'm not sure how the guy around Houston builds them that cheap and make money.. Just look around local.

3rd option is a salvage yard it will cost around $1000 they are not going to last as long as a rebuild (or they can run forever)

its up to you depending on money and skill level.

Just try the pan first and check the pickup tube.