Hi, even though I cant really help with your actual question, I couldnt go past your post without leaving a response.
I live in the Adelaide Hills, not far from Mt Barker, but although we have horses I am not involved in the horse scene as I just ride for pleasure, and I agist at a friends place a couple minutes from my house so not really sure about agistment either.
As far as trail rides, I guess it really depends on where you end up living. Beach rides obviously would be great if you are near the coast, Kuipto forest which is kind of more southern hills is lovely; Mount Crawford is about an hour from Mt Barker and is where we go for forest rides. Or if you are agisting in the hills there may be rides you can go on from your agistment property, which is what I do from my friends house, going through orchards and vineyards etc.
The only agistment place I know anything about is the one I go to for riding lessons, which is Oakwood Equestrian in Birdwood in the hills, Front page - www.oakwoodequestrianacademy.com
The owner/instructor is lovely and horses are well cared for. I really dont know how prices compare to other agistments here or interstate, so cant comment on whether you would consider it cheap, it is NOT close to the city, but you can have paddocks with turnout sheds and do self care.
Hopefully others more in the metro area, and more into the horse scene might have some info for you, but just couldnt go past the post without at least responding. :)