I now have a 2001 K2500 suburban. 180K, 6.0, 4L80E.
Runs great, Rust free. Does not use any oil. It had a few things wrong when i bought it, but i have spent under $300 fixing it all. Parts are available anyplace.
The GMT800 is a small upgrade over a GMT400. Little secret, Here in the PNW rust is really a non issue. People compline about how the stuff they put on the roads causes all the rusted cars/truck. Not true really. There are FAR more rust free vehicles around here then rusty.
If you have a good running rust free, good interior GMT400, keep it. If you want/need a newer suburban, a GMT800 is a small