why not just bite the bullet, get a screwdriver and scrape off the rust, assess the problem ,if it's Swiss cheese, pull the shock ,grind out the crap till clean solid metal and repair it
OR pay someone the £100
to snot some metal on
clean it all up first and you will know the extent,and if your having it welded by someone, they will see what is involved and give an accurate price and not have to spend time cleaning up or over quoting for unknowns .check the other side too as DG says
if the rest of the car is serviceable, without issues , saves hunting down a replacement vehicle, which will probably have issues and need repairs and money spent.(plus, your obviously comfy and happy in an Omega ,saves swapping insurance etc etc)
as Nick says, it's not an area you can even see, BUT ,is an area of a car that needs to be strong ,more important than a bit of crusty outer sill
it does not have to be pretty , it does need to be strong
or look for something with 12 months MOT when this car becomes un-economic to repair ,when the MOT is due again or the shock tower is unsafe ,which may be now , so , assess the rust ,rather than just looking at it