[Ethereum] nything stopping me from using the ethereum blockchain as data storage

I think you can manage it with one smart contract. Without knowing your exact requirements, my approach to the scenario you have described would be as follow.

There should be an id to uniquely identify a document. Set it to an integer. And document hash can be stored as a string value. A mapping defined like, mapping(uint => string) documents can be used to store the hashes.

To use to store a hash define storeDocument and to verify define verifyDocument functions as follow.

pragma solidity ^0.4.0;

contract Example {

    mapping(uint => string) documents;


    function storeDocument(uint id, string docHash) {

        storeDocument[id] = docHash;
    }

    function verifyDocument(uint id, string hashToVerify) returns (bool){
        if(storeDocument[id] == hashToVerify ) {
            return true;
        }
        else{
            return false;
        }
     }
} 

Now you can store and verify the documents calling the contract's functions.

NOTE: This is a very basic example and you may need to design the contract according to your requirements. Like you may need to avoid replacing the document hashes if there's already a hash, or you may need to check who's storing the hash etc.

Hope this helps!