Which old engines??

For the electrics any one of them, 3406E, N14 or Series 60. Maintain them properly you have a good chance of getting big miles out of the first gen electronics. Ones really no harder to work on than another. Where they tie your hands a bit is either service tools or access to information. Once you find the work arounds for those they’re all pretty much the same.

 

First off, define "old". There are technicians on here that have a very different point of view of old, from some of the mechanics, old school.

 

I'll agree with @kranky1 for a 90's engine. All three were good. IMO, 60's are easiest to work on with reasonable parts prices. Best fuel economy here.

3406E 2nd easiest to work on. Parts are pretty expensive. The best powerhouse in a stock engine, and probably the best longevity.

N14 3rd. Hardest to work on and a bit tougher for those w/o experience. Parts prices are reasonable.

 

The older you go the more odds and ends that will need replacement. You'll go to do a minor repair and find a handful of airlines and electrical that need replacement. Same thing with coolant, fuel, oil and powersteering lines.
Very few pre 2000 trucks are turn key ready to work hassle free without being one frayed or sun dried hose away from needing repair or getting sidelined. It's relatively easy to replace lines and electrical but it's a good deal of work and absolutely necessary for peace of mind.
If you get a proper specced truck for your application then any of the engines described above will serve you well but the plumbing and electrical will be your weak point.
Personally I'd look for a 3406b Cat but I'm partial to fewer electronics.

 

I like the Series 60. Check out U tube for things to look for on a Detroit 60 by TAT Express. Also, check out Blue Ribbon Logistics video: Episode 81: Choosing The Right Truck for Your Operation

 

despite what others will say about downside of old trucks…I don’t see many lol. If you are fairly competent in the shop you will most likely come out ahead with old iron. I don’t know how fast you need to be operating or how much work you can do but I would try to buy something with a known problem because it’s going to be cheaper and then figure on rebuilding and replacing old hoses lines air bags and rubber bushings. With the goal of minimal issues once you start running. We have done very well with this mindset over the last 20 years but also have a very capable shop. Wiring might be a problem depending on Mickey Mouse and who’s worked on it. some trucks will literally have 4 splices in a foot of wire. i know mileage might be a little worse but big hood trucks are generally much nicer to work on because if better access to stuff. Older stuff tends to be built with the idea of being rebuilt (thinking nuts/bolts instead of plastic snap together crap) N14 can be tricky if you talking celect or celect +
not bad but seem to be very variable on how they run
Personally I would run 3406b/c or big cam…both can get respectable mpg if you treat them right

 

I tend to agree with what your saying .. but I'll add this.. You better have a good shop and tools and don't be suprised when the unexpected rears its ugly head.. My general rule is you better have twice the funds you paid for the truck .. to spend on fixing the truck .You also need to remember that a 30 yr old hood has probably changed ownership 3 to 5 times maybe more so it's been cannibalized 3 to 5 times maybe more usually the last 2 owners have just patched it together and not fixed things correctly causing issues that are unforeseen
I can't begin to count the times I've fixed problems correctly on a truck only to find the correct repaired has caused other issues to show up.. So just be prepared.. to deal with what looks like a easy 100.00 fix may end up being a 600 or 1000.00 or more fix when it's all said and done
The element of suprise never fails to show up when taking on a old iron truck