If it was me, I'd just go ahead and plant grass seed. Anything else can come later. I don't see where the HOA can force you to do anything else. Everyone has a right to have their lawn the way they want. IMHO So if you want nothing but grass, then so be it. Anything else can be added later, when you have money and have decided on a specific plan. If your yard is eroding into the neighbors, you may need to put in a retaining wall, but these can be as simple as a stack of railroad ties or some of those keylock (cement) blocks. Something you could do yourself. Go to your local library and take out a book or two on landscaping. Most will have some type of "How-to" on putting in a simple retaining wall. Good Luck. Lady I, would come help install it if she could.