Which freewheel/cassette for this bike? I was recently given a mountain bike for free, which I could fix for use by my husband. It is just missing the rear wheel and freewheel/cassette. As far as I can tell it would have been a 7 speed one,as there are seven gears on the handlebars (correct me if I am wrong) plus the three on the front crank.
Could anyone tell me how I would know if I need a freewheel or a cassette, or if it matters? For info:

26 inch 36 spoke wheel on front
Shimano SIS rear derailleur
Shimano altus front derailleur
SRAM twist shifters I think
it will just be used for laid back family rides
it is a Explor brand made in 2000

Everything needs cleaning but seems to work. It is a bit modern for me! My other bikes are all 70's and 80's so it will be fun to do something new. I have never done indexed gears before 
Many thanks for any advice.
Gill
As you are starting from nothing, it doesn't matter whether you use a freewheel or a cassette. The 7 speed chains are the same regardless. I would look around for a secondhand wheel, as it will be much cheaper, and you'll want one that comes with cogs, tyre etc.
If you are lucky the chain isn't too worn, and it will just work. You may, perhaps, need to adjust the gears as the offset might be different