Personally I would definitely choose a steel frame for durability and the repairability issues mentioned. No need to worry too much about strength here, welded frames should be fine, but stiffness is a priority. Not all old, tough steel frames are stiff by any means and it's possible to end up with the worst of both worlds, so choose something with stout stays if possible. Wheel strength and reliability is likely to be a much bigger concern than whether the frame's tough enough.
I don't know whether the inconvenience would make it prohibitive, but a BOB Yak trailer is a serious alternative to panniers. The single wheel trailer handles much better than a pannier-laden bike and doesn't require the bike to have a suitable geometry to keep the ride stable, though a stiff rear triangle is still important. It can carry huge loads up about 80lbs, which enables light but bulky items to be taken which would be impossible with panniers. Also, as the load is shared between the trailor and bike, a lot of stress is relieved from the rear wheel. I would go as far as saying that a much lighter bike can actually be used with a BOB as long as it's stiff enough and still has strong wheels.
One last bonus is that if you flip it upside down it makes a surprisingly useful table!
I switched over from panniers to a BOB years ago and have never looked back - the handling is that much better.
I don't know whether the inconvenience would make it prohibitive, but a BOB Yak trailer is a serious alternative to panniers. The single wheel trailer handles much better than a pannier-laden bike and doesn't require the bike to have a suitable geometry to keep the ride stable, though a stiff rear triangle is still important. It can carry huge loads up about 80lbs, which enables light but bulky items to be taken which would be impossible with panniers. Also, as the load is shared between the trailor and bike, a lot of stress is relieved from the rear wheel. I would go as far as saying that a much lighter bike can actually be used with a BOB as long as it's stiff enough and still has strong wheels.
One last bonus is that if you flip it upside down it makes a surprisingly useful table!
I switched over from panniers to a BOB years ago and have never looked back - the handling is that much better.