First things you want to do to that 6 leaker are:

Change the oil and filter. Don't trust anyone to have used the correct OEM filter, or even the correct oil. The correct oil is expensive, and many will use gasser oil to save a buck if they know they are selling.

Change the fuel filters (2). Again, only use OEM.

Don't worry about the air filter. It will tell you when it needs to be changed.

Before you tow, install gauges for coolant temp, oil temp, tranny temp, and exhaust gas temp. Easiest way to do that is an Edge Insight.

I use 16oz of Diesel Kleen with every tank of fuel. That's been good to me so far.

Your tranny needs a fluid flush every 30k.

Be prepared to do a lot of grinning while towing.