FIP does not have to be as scary as it sounds. While the threat of transmission is possible, the threat of symptoms arrising is low. MANY (MANY) cats have been exposed (possibnly including the Siamese you are concidering), but unless the virus mutates AND comes in contact with a feline with a compromised imune system, they can remain healthy.

I have had foster kitties, who after being adopted into their new homes, died from FIP. That means that they were in contact with the other cats in this house while carrying the virus, but no one has ever become sick because of it.

The decision is ultimately yours, but do not allow fear of FIP to prevent you from enjoying the company of another needy feline. (AND, most importantly, IF by any stroke of fate your next kitty becomes ill, remember that s/he could have been sick prior to adoption and it will NOT be your fault!!)