Lots of good advice here. A vacuum gauge in the car is the best way to monitor and engines health. While its great to note oil pressure as we all know you can loose rod bearings and never see oil pressure waiver . If you start loosing vacuum you have some issues.

Those plugs look like HELL why is the question, I agree start with a compression test and then if you must follow up with a leakdown. FWIW 8% on a cold engine is pretty damn good, but it dont mean a lot without the compression test, especially at 8%. AS bad as those plugs appear(looks like oil but pictures can be deceiving I suppose) I would be digging a bit deeper. If its got valve seal issue and was started up and shut off a few times they could get ugly quick, especially depending on heat range. But oil and nitrous dont play well together so I would definitely want to get handle on it.