I know it's an old thread, but maybe it will be a timely resurrection for someone
We looked at metal ones but many of them seemed a bit ricketty and I couldn't imagine them standing up to hard use.
Also suggest when you buy looking at how difficult the beds will be to make e.g. one that has a ladder on the side or is hard to pull out from the wall will make it harder to make the bed. I've seen some where I think the only way you could make it would be to climb up make it while clambering over the mattress and ain't nobody got time for dat.
We also got secondhand Bunkers like this types https://10restbest.com/best-storage-beds
(new is very pricey) when my DSs were about 9 and 7 and my now 22 year old still sleeps in the top bunk. Uses the bottom as a clothes pile except when friends stay over and sleep in it. The end of the bed has a couple of solid boards across it that can be used to climb to the top bunk, but the bottom one is missing now after a tall mate unintentionally kicked it off while sleeping in the bottom bunk one night.