Mountain bike with questions.
 

This bike has a nice feel and ride to it but has heavy steel wheels and a rigid front fork. I'm going to swap the wheels for a used set of alloy wheels but need advice on what would be a decent adjustable inexpensive front fork to put on it. It's a bit of a beater with a good solid frame but worth upgrading if I can keep the price low. I paid 25 bucks for it, so not much invested so far.
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If you are not planing on changing to disc brakes, you will be able to find many older but lightly used cantilever brake suspension forks on Ebay or Craig's List.

You should be able to find new Suntour 80mm to 100mm for under $100. They are not great, but not bad for the price.

Threaded headset, you need a threaded fork. Most likely 1 inch steerer. Try eBay, but most of those are going to be old and crappy.

I doubt that these wheels are steel--Just aluminium rims!

They are steel. I checked them with magnet.