My first raised beds were bought off the internet.
I was pleased with my purchases as at the time although later realised the materials could have been bought at a fraction of the cost at the local DIY store or reclaimed material place on a local industrial estate.
Scaffolding planks look ok with wood preserve on although I've only used these around the GH as a frame for the base to sit on.
My raised beds are made form decking planks, painted with wood preserve,not as durable as sleepers but don't look any different now from the day they went down several years ago. The most recent bed cost £7.50 to make, think it's about 6t by 6ft but only one plank in depth any higher and I'd have lined it with plastic to preserve the wood.