Hi Tracey, Kimberly here from landscape solutions for you. Thank you for asking your question on planting on an entire slope. Living in zone seven in the United States we have very clay based soil. My crew and I have done many jobs on slopes and have run into the same problem as you have.
Preparing the bank can be tricky as many times there is no way to use a tiller or even shovel, especially when there isn't much soil. You are right when you mentioned bringing in top soil. Add three to four inches of top soil across the bank. You can do more if it will hold. Jute is a great commercial landscape fabric that is especially made for slopes. Jute is made out of burlap and helps with erosion and plants staying in place. If Jute is not available in your area, check for a similar commercial grade fabric.
As far as plants for zone five on a slope, here are a few suggestions.
1. Junipers are great and many varieties will grow in your zone. Blue Star, Blug Rug, Parson, Sea Green, and Green Mound. Sea Green and Green Mound are larger shrubs and will give you some height and width.
2. Cpyress will also work. The Mop Cypress is a great shrub with variegated leaves of green and yellow. Dwarf Hinoki is another that is solid green.
3. Hollies will do well. Shrubs such as Soft Touch and Helleri add different texture and color to the slope.
4. Arborvitae - Globe and Rheingold are two examples.
All the above are evergreen and once established will grow well. Add organic mushroom compost to your new soil. This is an organic fertilizer that will last up to one year. Put the mushroom compost in each hole you dig.
These are just a few examples for you to think about. I hope this helps. You might get some more varieties for you slope looking in the gallery of plants section. Also here is a page on slope planting with evergreens that may help also.
The best of luck to you and let us know how you make out with your slope of hardpan. If you need more information, feel free to contact me here at landscape solutions for you. Ask a question anytime. Thanks again and have a great day.
Kimberly