Miles aren't that important. It all depends how it's looked after. My old 5.7 vortec calved at 373'000 kms, but it had a hard life. I think hours would be a much more accurate measure of wear than miles, but it still depends on how it was used. That said the most beat up and abused '400 I've ever seen (that I got for parts) had 438k on a 4.3 and still runs great.
There is no realistic "average". Engine failure happens for a reason. Sometimes it is old age, but I've seen trucks on their original, non-rebuilt Vortec with well over 600'000kms.
Now, there are common problems with these engines that will almost certainly need to be addressed sooner rather than later if they haven't already. Intake gaskets come to mind right off the bat. That and the spider injector assembly, I'm sure others will chime in with other common problems. But neither of these are jobs so huge to be too afraid of, even with limited mechanical experience.
The engine certainly wouldn't be my biggest concern. I would be considerably more worried about the condition of the (10 bolt, I presume) rear end and (4L60E, I also presume) transmission. Both of these are known weak points on these trucks, and unfortunately, unless you see something obvious (leaks, burnt transmission fluid), they can be almost impossible to judge. If the truck was ever plowed with...good luck my friend. Just sayin'.
However, these trucks are extremely easy to work on and parts are not only cheap, but relatively abundant. No matter what you buy, stuff is going to break, eventually. Period.
