[KTL]

Most engines show significant wear on the IS bearings, even with low miles or even when opened up after a recent rebuild. It's just a mild shortcoming of the engine structure. That said, IS bearings really don't fail that often, if at all. Main bearings are often reusable. Rod bearings should be replaced since they take a TON of abuse. C-C length of the rods should be checked and the big end bore. Rod bolts are simply replaced with new OEM or aftermarket reusables like ARP. Rod inspection can be done with a bore gauge and quality set of calipers and micrometer. Any competent machine shop can check the rods against specs and magnaflux.

I find it odd that people think the top end is the easier of the two, top end vs. bottom end. Bottom end can be opened, visually inspected, and put back together with little worry of how it well will function when reassembled (assuming proper assembly of rods and sealing of case). Everything just drops in there and you seal it up. There are some tricks, such as case thru-bolt o-rings and ensuring the #8 bearing seals properly. But on the whole the bottom end assembly is pretty straightforward aside from the rod inspection. Crank inspection is visual, a simple check with a 1"-2" micrometer and any machine shop can magnaflux it for you.

Top end is more critical IMO when you consider the machine work needed for your heads and the concern of new rings sealing well if you're reusing your pistons and cylinders. Also need to measure piston ring clearances to ensure the ring lands are not worn. There's new parts often needed on the heads like new valves and certainly need new guides. Then there's the head assembly (setting proper spring height) and cam timing. Let's not forget rocker arm bushings, rocker shafts, rocker shaft bores in the cam housing....... There's a lot more pitfalls in the top end IMO.