unfortunately GM dropped the 4.3 replacement crate engine.
Running one with bad oil seals will not cause any extra damage.No more than the wear that comes with 200,000 miles does.
Depending on your machine shops in your area you are looking at $1,800.00 and up to rebuild a V/6 back to stock.
I am personally not a real big fan of Jasper remaned engines.

I have also seen ford style rubber boot/umbrella style replacement valve seals used on Chevy engines...

......Wookee

You may be able to get away with just replacing the valve seals in which case the heads don't need to be pulled....

remanufactured engine

Just replace the valve seals, use compressed air to hold the valves up, simple job for some.

on the valve seals, if it aint using oil, dont waste time and money on it, it s not worth the trouble and it isnt hurting anything.

Stripes- buy an eraser wheel from a body supply shop, only way to remove those stripes with zero chance of damage to the paint. Use that along with a corded drill, you can do the whole van in a couple afternoons.

my 92 g20 had a smoking problem too, and being i dont like second hand smoke i found the problem. pulled my valve covers and found the 75% of the return holes were pluged up. that caused the oil th sit up there and get forced down the seals(not saying the seals arnt worn). now no more smoke at all. something to look at. good luck friend