How much steerer tube length do I need on my suspension fork I have survived BB hell with minor bruising to be greeted with another challenge replacing forks. 

I have a 1996 TREK 8000 SHX (yellow) and a 2002 TREK 4300 (silver/blue) MTB bikes. Both bikes have 21" frames. I want to get my wife and I into trail riding and I am looking to replace the suspension forks. 

As beginners, I do not want to break the bank buying these forks. I know a crucial point is getting the steerer tube length and diameter correct. What are my considerations? Is there a way to look up the lengths of the tubes for 21" frame? I do not have the physical old parts.

Thanks in advance.
No, there is no way to look up the lengths of tubes. According to quick web search, both bikes have 1 1/8 threadless headsets, so that is the diameter you should shop for.

What you can do is measure the head tube of the frame, add a couple of centimeters for headset, around 5 centimeters for the stem and room for enough spacers to get the handlebars at the height you want.

Typically threadless forks come with very long steerer tube to fit the largest frames, and you are supposed to cut it to correct length yourself. When in doubt, leave some extra length and fill it with spacers until you are sure that you want to keep the bars at lower height.