I have had these old Land Raider models since they first came out in the Space Marine boxed set. Back in the day I painted them silver, applied a brown wash to represent them being rusty and old, and called them Death Guard land raiders.
These days I dedicate a little bit more time to Death Guard models when I paint them, so spruced them up to match my contemporary Death Guard 30k (and 40k) theme.
I'm also basing my models these days, so added the land raiders to their required 33mm steel washer covered with sand.
I painted them up in the same colours as the rest of my force, with an off white base, green accents and red weapons. Tracks and other hard working parts are painted brown, given a light metal brush on the raised bits, and then the whole thing is given a liberal brown wash. Final highlights are picked out in white, and I'm good to go.
I haven't added unit markings or numbers yet. I would like to see how far I go with them before I do. It is also a problem when I use different rule sets, as formations have different sizes (or variable sizes) in different versions of epic.
And for twenty+ year old plastics they don't look too bad. Their styling still fits the current narrative that is being published too, with the Land Raider Proteus still being supported in the 30k/40k universes. So I'm glad I didn't chuck them out!