I have a product that does 5 tests for a certain price which I want to be available for the general public. For my wholesale distributors, however, there is a special trial version of this product, at a lower price.
how can I make this so this product will only be available to the wholesaler (and not by regular customers)?
I just can't figure this one out...

Try this.
You create two products in the parent category.
One is the public version, the second the wholesalers.
 
Go to CMS>Site Navigation>Tree View
Tick the public one to "Show the menu item for NOT logged in visitors"
Tick the Wholesale one to "Show the menu item for logged in visitors"
 
Now a Wholesaler will have to log in to see the product and it wont be visible to the public who will see the other one.
HTH

doesn't this mean that all registered customers can see the product?
I was more thinking along the line of trying to create an extra option in the general product information fields.
for instance, there is an option that selects a product as viewable only for certain languages
(in admin_product.php:
$languages = get_db_values("SELECT language_code, language_name FROM " . $table_prefix . "languages ORDER BY language_order, language_name ", array(array("", "")));
$r->add_select("language_code", TEXT, $languages);
$r->change_property("language_code", USE_SQL_NULL, false);
)
 
I don't think it should be too difficult to change this to make a selection between users.
 
My gf is pregnant with our 3rd baby and she's got pneumonia at the moment. Taking care of the household and running my business is preventing me from really getting into this.
I would appreciate it if someone would think about this and say whether this would be feasible.