MTB fork

Hello,
I normally ride road. Uphill.
On my MTB I even changed from rigid to suspension fork. Old fork sold on eBay.
But right now, at the tender ripe age of 50, I entered my 1st ever MTB event. So I want to move back to suspension fork.
The MTB is great (full carbon, top specs), in perfect condition, though 10yr old and 26".

What is there to know about suspension fork? The price difference from top to bottom is very huge...

Are there fork that I can make completely rigid? When I still had a suspension fork, I hated that uphill it was always sucking energy... I know one can make it stiffer/softer, but that's not the same as making it completely rigid.

Thanks,
Alberto