I concur with the above posts - unless you have a specific reason to suspect that the pistons or rings have a problem, I'd bet that the bottom end of your engine is in good shape. You'll more than likely be able to fix your issues with a top-end rebuild - I did that at 130K miles on my 3.5 and with over 230K on it now the bottom end is still in good shape. You can remove the heads and everything else without having to remove the engine from the vehicle if you're just doing the top end. If you have any oil leaks from either of the oil pan gaskets, or if the leak-down test that you're going to perform indicates seriously worn rings, then it makes sense to look at doing a complete rebuild, which will necessitate pulling the engine.

Here's another parts source to consider, especially for the valve lash adjusters:

http://www.store.partsdinosaur.com/page213.html

I usually get my OEM parts from Southwest Mitsubishi in Texas:

http://www.oemmitsubishiparts.com/