When looking at MGBs, you need to decide if you are looking for a perfect car for a restoration or a car that has been "restored" and you are judging how well that has been done.
First place I look is the pinch weld and what is known as the "castle rail". What do the spotwelds along the pinch weld look like? Are they factory or are they puddle-welds made by the person who restored the car? Original means the sill has never been replaced. A rusty castle rail is a good sign that the car has rust. A rusty castle rail and a fresh painted car is something you want to run away from.
Now go look at the rear wheel openings. With a glove on, run your hand around the inner edge. Does it feel thick? Did it slice the glove open with a ragged rust edge? Look at the spot- welds again; are they factory or repair quality? Are they well done?
Look at the rocker panels. There should be a line at the rear and front just below the door where the rocker meets the rear quarter and the fender at the front. If they are not there run.
Look at the beading at the top of the rear quarter and the front where the fender meets the scuttle. Any rust that gets into that beading is very hard to eliminate.
If the owner claims new floors, make sure they cut out the old ones.
Check the front and rear spring mounts for rust.
It the car has quarter panel repairs, look in the trunk to see how well they were done. I like to butt-weld the parts on and make them invisible, but that is too much work for most people. At least get and idea if the were sloppy with the cutting and welding.
Dead give-away that the sills have been done are the spotwelds at the base of the a-pillar that attach to the sill. Very hard to replicate these with a factory look when doing sills so just open the door and have a glance.
Look for wrinkles in the front frame rails right around where they attach to the crossmember. At the rear, look at the framerail at the top of the wheel-well an right inside the trunk panel.
I would stay away from 3 synchro gearboxes unless they have just been rebuilt. The first gear is not synchronized and at best sounds like the box is grinding a load of gravel. They only get worse with age. The 4 synch is bombproof.
In case you want to know how the sill area is designed, and why it rust out so efficiently, I did this sketch several years back that shows how it all goes together or comes apart if need be.