Mountain Biking - Which measurements are important when choosing new crankset?

I need a new 3spd crankset for my mtb but I don't know what measurements I'm supposed to take into account, apart from needing a JIS square taper. Is spindle length important to the chainline or will any JIS properly incorporate onto the bike? The other factor I was wondering about was the number of cogs on my freewheel. Does having a 7, 8, or 9spd effect what you can put on in the front?

The mtb has been converted to a tourer and the freewheel is only 7spd so I need a low geared crankset, something like 22, 32, 44. The lighter the better and I'm looking for something $50 or less. Here's an example http://www.jensonusa.com/store/product/CR307A06-Fsa+Dynadrive+Sq+Taper+C-Set+Crankset.aspx

If anyone can suggest a good crankset that would be great. Thanks in advance for any help.

Non-technical answer: I think you'l be OK if you do't exceed the front and rear derailler's capacity. I assume this MTB bike had a triple, so you are in the ball park, but you might look the front derailler's specs up on the internet and make sure it can handle the difference between your proposed largest and smallest chainring. Otherwise, as long as you are going MTB triple to MTB triple, same bottom bracket and spindle, I don't think you'll run into a problem. You should also be intentional about selecting the crank length; big feel difference between 170mm and 175mm, imho.

thanks for the advice. I did some reading on Sheldon Brown's website and it looks the crankset I posted in the link should do fine.