The simplest, least expensive, and most effective method to clear a planting area of unwanted plant growth is mechanical, just dig those unwanted plants out. As a general rule of thumb spending your money on something called "topsoil" is a waste since "topsoil" can be anything the seller wants it to be. I would hazard a guess that you think the "topsoil" you would get would be loam and it will not be.
What that soil most likely needs is organic matter and even iof the soil you buy has 5 to 8 percent organic matter in it that still means the soil you have needs more organci matter to bring the level of OM in that soil to the desired 5 to 8 percent. Look for compost, or leaf mold, instead of "topsoil".

If you use Preen you can only plant plants. Since it provides a surface barrier to prevent seeds from growing I don't know what it would do if turned over. Sounds like a really bad idea. Remove the ivy by hand and use the stones to provide a shelter for toads. Some ivy killers will remain in the soil for a long time. Depending on what you want to plant many flowers will be killed by frost later this year so think before you spend your money.