I have a old school 1993 mongoose rockadile with a shot duotrack fork. I plan to replace with a rigid fork. How important is replacing with a suspension correct fork vs any old school steel fork?
range for good fork is $60-300......I am leaning towards the cheaper side of life.

I did replace the now gone elastomers with some foam from my kids noodles used for swimming now have about 1/2 of travel until they explode!

How badly does your fork sag now? If your fork is perpetually in compression you probably won't feel any difference at all.

I started with a new frame that was intended for a 50mm travel fork and used a rigid fork.

I looked and looked, the Dimension fork was the best deal in a rigid, non-disc fork that I could find. I was surprised and the finish and fit that came on a $50 fork.

I never used the "proper" fork on this frame so I can't comment on the change this rigid fork made. I do know that I like the way it handles and it rides fine, so whatever the difference, it means nothing to me. I'm happy.

it has zero suspension.......so the fork can move up/down without any friction.
I have read suspension specific rigid forks are out there just did not know if back in the day 1993 was a part of the design?

You just need a suspension corrected fork to replace the suspension fork.

If it moves up and down without any friction, then it will be compressed all the time when you ride and a non-suspension corrected fork will steer about the same. I'm thinking that a suspension-corrected fork would handle a little nicer but it's not something I would worry about excessively.

If the blade length is kept the same, .. fork tip, to crown race seat, then the steering - head angle

won't change .. [shorter, steepens the angle] .. hence the, suspension corrected, fork ... longer blades..

It moves up and down with no force, or no friction? No force means no or broken spring (coil or air). No friction can mean no damping.

I'm lucky: the guy who gave me my Rock Mountain included the original, never used, rigid front fork. So if the primitive Rock Shox ever die on me I'll put that fork on. Those Rock Shox don't have a lot of suspension play, but they sure have held up well over the years. They never seem to lose their air pressure.