Hi,
I am a qualified riding coach and horse trainer in the ACT area. I have been breaking and training my own horses for about 25 years. I have been riding for 40 years.
I would strongly suggest you find a qualified coach with the EA (Equestrian Australia) and check out some of the local pony clubs. For the type of riding you want to do I would strongly suggest older horses (over 10 years) with experience of the type of work you want to do. Riding schools usually give a very good grounding but owning your own horse is completely different because the "one owner horse" gets heaps less work and repitition than the average riding school horse. In other words what is easy on a good school horse can be quite a challenge on your own horse.
If you are keeping two horses on 3 acres I would suggest you use stabling in the evenings (or reverse stabling in summer) to rest and save the paddocks. As mentioned above Jane Mayers also has some great ideas. I keep my three horses on 5 acres and I do this and manage to maintain a certain level of grass even though I am officially overstocking my property. The other good thing about stabling is you maintain a routine with your horses and you can monitor their feed intake which is vital for ponies (they can get too fat and founder easily)
Costs vary.
In our area the cost for one horse that is in light work being kept as suggested on a small space would be as follows.
Weekly
Feed (small hard feed and some hay daily) approx $30
Every six weeks
Farrier for a hoof trim (well managed bare feet are cheaper and in many cases better for the horse) $40
Worming paste $10-$20
Yearly
Vet/equine dentist for teeth maintenance and general health check $120
Incidentals
Rugs (I usually buy good quality ones that fit the horses well, these last about 5-6 years) $200
Additional vet costs for illness injury $200 (this is based on my well managed horses/per horse over the last 15 years,,,,,keep in mind it can be very very expensive for some treatments)
Average lesson cost $50-$120 for an hour. I know some people who pay up to $200 for a lesson and others who have got friends who help them for nothing. Good coaches have insurance and also can advise you on ongoing horse care and other management issues.
Good safe horses have no price. I personally think a well bred and trained horse for $5000 is pretty cheap. That is in fact slightly less than the cost of production for a broken and working well behaved horse.
I hope this all helps,
Good luck on your equine adventure!