You can usually clean fungus successfully. Only problem I've encountered is that it seems to have a fondness for the glue used to hold multi-element groups together, if it gets into those then there's nothing you can do to clean it.

I'd get a quote from a professional to sort it out (they should be able to fix the filter ring too), then compare that to the replacement cost. Zooms are harder to work on than primes but can still be cleaned - I've worked on three so far, two had a single bloom on one element while the third was literally riddled (I had to remove and clean every single piece of glass). Dry storage and frequent use (exposing them to UV light) seems to keep it from reappearing.