We're looking into this ourselves right now and have found Demandbase & Terminus to be top contenders. DemandBase has a great feature for its inbuilt account based advertising and could be very useful when we further go down the road of personalization.
Marketo ABM Option will cover all your needs except for add buying.
Hi Allison Rudy,
I have had the opportunity to work on Marketo ABM with several clients with varied goals and strategies. Most of them had great success and experience working on Marketo ABM. Like Greg mentioned, except for add buying, Marketo can flawlessly execute all other requirements. I would highly recommend it.
Demandbase is certainly a cool tool as it has great user friendly features, my recommendation will be based on the size of database you have. If your CRM or Marketo database have good number of people from the Accounts you are planning to target then Marketo ABM will be best. Incase you need to look for people as well then Demandbase will be recommended, though you need to be careful about the GDPR and CANSPAM rules.