With the tyres you could line them I guess... with some punctured plastic for drainage... I was just trying to save you the cost of sleepers/metal supports. I know the tyres are not very aesthetically appealing... maybe you could cover them in hessian?

Have you investigated how much popping in a raised bed would cost yet? Good quality compost soil is over $100 per cubic meter alone. Could you use your own soil? Plant directly into the ground?

ETA: also ideally you mulch the garden to retain moisture and keep down weeds... another expense but worth it... we used pea straw mulch.