To replace the front seal you have to remove all the crank pullies, timing belt, etc, just to get to it.

To replace the rear seal, you have to removed the oil pan, transmission, clutch, and flywheel to get to it. Changing the seals themselves isn't difficult, getting to them is what makes it a royal pain.

As for the oil pan, distortion caused by over-tightening can also cause a leak, so resist the urge to wrench down on the nuts unless they're just way too loose. If you're going to replace the main seals, I would highly suggest replacing the oil pan gasket at the same time, as well as making sure the gasket surface of the pan is flat. You can knock down high spots by supporting the pan on a work bench and tapping with a flat hammer.