Areas that rust - Front panel lower
Lower Front Panel Rot
Covkid: This is a common area for corrosion. Although this section can look bad, its usually just the front panel that rots where it meets box section behind bumper - wholly due to the fact that any water tends to sit there and has nowhere to go, eventually getting under the join and causing the lower panel to rot out. Obtaining this panel may prove difficult (check A. Schofield as a first port of call) but the damage may not require as much new steel as you think and can be repaired in most cases as follows (depends how long you've left things!).
Heres a video from 'Covkid' and 'Chickenkoop' showing an easy way to repair this section using flat steel, bent at right angles in a vice or a preformed right-angle section available from panel suppliers, drilled and puddle welded to complete the repair. You should spend time cleaning back to clean metal and treat any rust accordingly. If you make up your own repair section, you only need to gently persuade it into a slight curve and if you do this in a vice, you'll get the curve naturally.
Tip: You'll need a larger grinder with a new disc to get the reach needed. A 4.5" disc grinder will not be big enough.
The box section itself can corrode too but its much heavier gauge steel and usually only gets mild surface damage. Once cleaned up, you generally find all thats needed is removal of affected parts of the front panel itself, grinding off the the section where it was spot-welded down, cutting out the rot back to solid metal and welding in a bought or made lower section as shown here.
If you want to improve drainage in the area, drill two or more holes into the horizontal section, just larger than a section of steel tube, taking these tubes through both layers so they exit at bottom, then making sure tubes are flush at top, weld them in. This will ensure any water which gathers has at least one or more routes out, minimising a repeat of the problem.