Speed enough to keep up with the patrol without slowing them down, but nothing special. Damage would be nice, of course, but the patrol isn't exactly lacking there.
The patrols would have a fairly large range and couldn't be expected to come back in for resupply very often, so being beefy and self-healing would be ideal. Swarms of weaker Undead could be useful, though the many Sunburst SLAs each patrol would have available from the large number of Plant-Imbued might make friendly fire incidents more common if the swarms are expected to engage targets at range.
Looking over Qyburn's menu of available Undead, the Bloodsharks look like a good candidate, IMO. Fast, tough, solid damage, and they have Fast Healing, so not overly reliant on outside support. We could put have a Necrotic Mold pilot in each one of them for added reliability. The Molds wouldn't cost anything if we use corpse HD to pay for them and the Bloodsharks are quite reasonably priced. 40 of them per patrol wouldn't be too much of an investment.