Need help not a baker

No, that is not a bad idea.  If you want 2 tiers and are not a baker, I would suggest asking the store to make a 2-tier undecorated buttercream cake for you.  You don't want the top tier to sink into the bottom tier, and that's where it can get tricky (dowels).  Not sure how much the store would charge, but hopefully it wouldn't be too expensive.  That would free you up to make the fondant decorations.

Are you making your own fondant?  Marshmallow fondant is very inexpensive and easy to make (just messy.)

Good luck!  Some of us made a cake for our child and became so hooked on baking and cake decorating it's not even funny ...you should see my closet of cake supplies.

Yes.  You can buy small containers of fondant in those colors at craft stores like Michaels.  Larger containers start at $24.00 each, but they would cover entire cakes.

You would need to put a cake board between the two cakes.  They also sell them at craft stores.

Depending on how much you think you'll use, you might check out the multipack with red, yellow, green and blue, instead of buying big packs of the individual colors. You can mix them together to make other colors too, maybe you can rework your design to accommodate the colors you have (you can paint using the Wilton color gel for the black bits if you can't get black exactly by mixing). Also, if you're going to Michaels or Joann make sure you use a 50% off coupon! 

if you really want to save money, why not make a large frosted round cake instead of a fondant covered tiered cake? you might find that once you start buying materials and tools that you've gone over budget. There's no reason you can't make a beautiful, festive cake from one tier.  Also, covering an entire cake in fondant can be difficult. You don't want to be sweating it out the night before the party, right?

I agree with kikiandkyle, pick up a 4 color pack with a coupon at michaels, and maybe a set of small cookie cutters. Just a tip, the top of a salt shaker makes a good circle cutter.

i think this could be a good inspiration...

http://www.etsy.com/listing/76055028/girls-elmo-inspired-fondant-cake-toppers?ref=sr_gallery_8&ga_search_query=elmo+cake+decorations&ga_view_type=gallery&ga_ship_to=US&ga_ref=auto1&ga_search_type=all

FYI my first birthday cakes were sheet style with whatever image i wanted, piped and filled with star shapes, just like on the wilton shape pans. 

Then I graduated to round cakes and finally tiered cakes. 

I have a minimum order, so my smallest 2 tier is $118, decorated or not. Fully decorated like the picture would be $126, and you'll spend more than $8 at Michaels. So, I want you to be prepared to hear a much higher price than you at expecting for this "plain" cake. If you have a tiny budget your best bet is to make it yourself. If you can't bake, the store brand mixes are still fool proof, follow the directions but add an egg. Use whatever pans you have, and trim to the size needed. Make American buttercream, a quadruple batch is about $12. The mixes would be about 10. A roll of Viva to smooth with is $2. Use a butter knife and credit card to smooth it. I think the primary colors fondant is $20. Start with yellow and add a smudge of the red fodant at a time to make orange. 

Someone on a computer who is familiar with the site can direct you to some great threads that will tell you every thing you need to know. JanH? Hello? She is fabulous at finding links. 

I warn you that cake decorating can get in your blood, and is a VERY EXPENSIVE hobby;-) and is also a lot harder than it looks!

Maybe you can cut the budget somewhere else, if the cake is that important? Just saying. 

The edible sugar sheets would probably be a good option too, I've never used them but I hear they can be brittle so it might be more work than you want. 

Find out how much the store wants to charge you for a plain two tier cake before getting too far in to your plans. It might be more that you're expecting, in which case you should give baking your own a go.