I found a VERY clean 04 ram 3500 4x4 cummins turbo diesel (5.9 i believe) its a single axle, long bed with "crew cab". the mileage is 180,000. the dealer wants 20,000 for it. No rust anywhere, interior is flawless. 1 owner and garage kept. My question is, is $20,000 too much for this? also what should i look for when checking it out? new to the diesel scene.. any thoughts or opinions would be appreciated.