Posted: Today at 11:42
I think you might get some plants for free on Free-cycle - try advertising on there as folk often need to sort out their borders or decide to cut down the area in cultivation because of lack of time or needing the area for car-parking space etc.
A good source of plants is to join Hardy Plant Society which hold meetings nationwide and each branch often has a plant table where you can buy stuff cheap at monthly meetings and there is the potential for swaps or buying plants from the visiting speaker if there is one.
Lavenders are an excellent weed smotherer once established and there are plenty of different varieties to choose from. They also root easily from cuttings. Lots of shrubs propagate easily from cuttings; I have done this from other folks' hedge trimmings before now and variegated holly trimmed into a standard provides some colour during the Winter months when not much else is on offer.
Topiary might work well in a big area like yours as you have the space to include big shapes if that is what you want. Yew and Box are the traditional plants to try but there are others worth experimenting with.
If you can afford it it might be worth getting a miniature version of a JCB on hire to scrape out the rubble as it would be back-breaking to do it all by hand. Alternatively a load of decent top-soil on top might help assuming you can find a reliable supplier who will not sell you rubbish.