Which size round pans should I use? I would like this cake to be two tiers. Another stupid question -for example, if I use the 12 and 10 inch pans, do I make two 12 inch cakes for the bottom tier and two 10 inch cakes for the top tier? My pans are 2" tall.
How about buttercream? How much? I will need a crumb coat and a thin coat to attach the fondant.
Last question - I need to make my fondant black. I don't like buying fondant but I did end up buying brown (they were out of black). Can I color it with black gel color???
Thanks!

.......for example, if I use the 12 and 10 inch pans, do I make two 12 inch cakes for the bottom tier and two 10 inch cakes for the top tier? My pans are 2" tall.........
Yes, tiers are usually 4" tall so you would need to make two 2" layers put together w/filling.

.......How about buttercream? How much? I will need a crumb coat and a thin coat to attach the fondant....
Most people use a full 'coat' of b'cream under the fondant. How much is needed will depend on what size the cake ends up being. Most likely one batch of icing using 2# of sugar will be enough - but you might need a bit more.
To serve 50 people one 12x4" tier is about right. If you want to make it two tiers then I suggest you use 10x4" and 6x3" tiers (for 52 servings). OR add a 6" tier to the 12"er - for about 60 servings.

........need to make fondant black....Can I color it with black gel ......

Certainly Good way to go....starting w/a dark color. You won't need to use as much gel.

A 10" and 6" is exactly 50 servings:
http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

You might need about 6.5 lbs. of icing if you also need extra for decorating.

This should answer all your questions:

http://shinymetalobjects.net/cake/calculator/cakulator.cgi?option=2&option=Select+Cakulator