I want to run an applescript whenever a certain person signs onto my Messages buddy list I want to run an applescript whenever a certain person signs onto my Messages buddy list

Exactly. Thats exactly what I want. Pretty self explanatory.

I am aware you can run applescripts for messages events, however I don't know how to check if it was that person who signs in.
There is a buddy became available event, that passes in a buddy object. You can use one of the attributes on this object to see 'who' it is (handle, name or full name, for instance).

You can attach an arbitrary script to an event, in the way described at bynkii.com