biggest concern: rust... and the fact that these are vehicles that are all getting fairly old
1- yes: pick your favorite poison... mostly prone to being British
2- as far as I know there is no drop dead mileage on a D: there are some really high milers that do great and there are some low milers that have a lot wrong with them... YMMV
3- better for what? I happen to prefer the manual, others prefer the auto, but "better" depends on what you want from it.