In your opinion would you buy an engine and tranny from a donor with miles or strictly new on each? Looking for an L83 or L86
Your going to have a hard time getting a crate production L83 or L86 they are not offered yet as far as I know. Currently the Gen V production LT1 is available through Chevrolet Performance and it is quite an engine, all the Gen V's are.
A production engine means it comes with the intake, coils, manifolds, etc.. ready to drop in. GM does offer long blocks but by the time you source the other parts it's not worth it IMO.
Part #'s for Gen V production truck engines are being released so I would expect them to be available soon which should drive down the used market prices.
The other part of the equation is a lift out engine can give you the harness, modules, mounts and other parts a crate engine will not so you have to add that in.
Now that we have done many Gen V builds were getting comfortable with the OS's, hardware and software. It's odd but some OS's work better than others and we have pretty much settled on the configurations through experience and lots of road testing. The Gen V 5.3, 6.2, 6 speed and 8 speed all have their own personalities. So at this point we are purchasing E92 ECM's 8 speed TCM's, harnesses, etc. from GM so we can support the kits like the Gen IV's.
When it comes to an LS3 I'd go new, they run just over $6k and you get a new production engine with a GM warranty. Wrecking yards know the demand for any aluminum 6.2 and jack the price up so you might save a grand over new, not worth it IMO. I have seen LS3/L99's go for good prices but there are often issues with them. With all the hype I've seen guys pay more for a used LS3 than a new one.
Now when it comes to the gen IV truck engines any of the iron used engines are a bargain IMO. They are tough engines and can run for hundreds of thousands of miles. The Gen IV iron 5.3 and 6.0 will set you back $2-2,500 and the aluminum 5.3 a bit more. The Gen IV truck 6.2's(L92, L9H and L94) went out of production a few years ago and are still holding their prices, I'd say $4-5k is a decent deal on one and they are great engines. Stay away from the L92 if possible, it's a good engine just 2007-8 OS's are difficult to work with.
It's important to work with a quality yard, I have built up relationships with yards that stand behind their engines so they are usually very good. We do pay a little extra for 10-20,000 miles engines from late model vehicles because it is lot less expensive to swap the engine just once.
After 100's of swaps I can only think of a couple bad engines we got and it was more like damage from shipping, or gummed up injectors and valve guides than an actual mechanical problem. If you find the right engine the used market give you very good value for dollar. If you want a new engine and the engine is available and your willing to pay for it go for it.