Just use some green stuff to make losts of bloby flesh and boils all over your marines and vehicles. Make sure they stand out and wash any exposed flesh over with green ink or something similar. From experience, tentacles (easy to shape from green stuff) make dreds and marines look very nurgly. It is not hard to paint green stuff without an undercoat so you do not have to re-spray the entire force and remove all the old paint. Unless you want to, Im sure then no one could have a problem with your army.
Make the armor rusty looking with some ink washes and corroded by wiping some pastic glue on armor plates and repainting what the glue eats away.
One pack of green stuff should be enough to do the whole army if is is not above 2k so the total price will be 8 USD puls any paints you need to slightly alter the scheme.