Forms - Adding Values And Searching A Directory Dynamically

I have a form, and i am trying to add the values of 5 fields into one javascript variable. Then i need an ajax function to search a directory for a php file made from these variables and return a "file exists" or "file doesnt exist" message to the form.

For example, assume the following values have been inputted into the fields;
Field1 (Manufacturer) - Kawasaki
Field2 (Model_Name) - GPZ500
Field3 (Nickname) - None
Field4 (Market) - UK
Field5 (Year) - 2000

Then javascript needs to add the values of Field1 - Field5 (and add a few bits)to create (for example):

Kawasaki_GPZ500_None_UK_2000.php

Then i need ajax to search a web directory for the file above, and if it exists return a link, or if it doesnt exist tell the user it doesnt exist.

im hoping that i can use ajax to do this search without clicking any buttons etc. unfortunately i have very little knowlegde of javascript and ajax and dont have a clue how i can go about implementing this.

i know how to do it by posting the variables to the next page and doing the check using php - but i would like instant validation as soon as the form fields have been entered.