Fork

Regarding play in my forks: If I see movement between the upper and lower part of the fork when the brakes are on, it is to me a good sign that I need a new fork. Is this condition at all dangerous? Or can I take my time in finding a replacement?

I'm rebuilding my 1999 cannondale f500 i'm changing the fork to 1 1/8, i'm looking for a good light xc fork with the same geometry, i'd like the fork to be 100mm, if anyone knows of a good fork let me know