I'm sorry but your vet is wrong and you need to get a better trained one. They are not contagious, they are believed to be viral by origin but have cancerous implications Depending on how bad they are they can be removed in various ways. I have had horse with sarcoids kept with other horses and they have never been 'passed on'
Are you sure this horse doesn't have warts - they can be transmitted to other horses but are not serious and can be removed easily
If the sarcoids are life threatening and the horse was sold 'sound' then you do have a case against the seller if they are a dealer. It should come under the trades descriptions laws