The engine will have no problems to 500k, it's the accessories and truck around it that will incur repairs that add up. Of the 1/2 dozen plus high mileage vehicles I've owned, including an 06 ram 2500 4X4 diesel purchased new (for $32k and insurance paid $18k when totaled out after 10 years and 150k), every one has required a cash infusion of $2-7k when the odo hit 100-150k, sometimes two such infusions. Now if I bought the vehicle used for $5-10k I can stomach the cash infusion and putting many more miles on the vehicle. If I have to pay $20k for a used vehicle that goes for $40k new, and will realistically have to dish out $5-10k in the next 100k miles it doesn't pencil out when I could buy the new truck for $40k, drive it for 100k trouble free miles and sell it used for $25k. The new truck will cost you at least as much as the new one for those 100k miles, but the used one could cost you much more and could leave you stranded at a very in opportune time or location.
That cheap commuter car is going to need insurance and repairs, so factor that into your cost comparison.
I understand the economic considerations, I have 2 kids in college and haven't replaced the totaled 06 ram yet as I'm going to give the oil industry another year to settle out before having to deal with a car payment for a new truck. But unless a very clean truck in the $10-12k range falls into my lap, I'm going new.
