Don’t go just by MOT history. Mt S-Special has zero evidence on the MOT history of any rust, but I had both sills and arches replaced, as they had rotted out. I don’t have spot welds on the arches anymore, but that really doesn’t trouble me. Some of those trying to reproduce these spot welds on the lip must be doing so so they can deny any repair had happened.
Check that the panel joint is visible below the door. Cheaper repairs using scrap tinplate will dress this in with filler. Check the lip for the jack. Its not part of the outer panel, but is formed from the inner sill.If its bent over, its not because someone has been careless with a jack, but because the sill itself has lost structural integrity. Some repairs will fabricate a lip in the sheet metal used for the outer panel. As the panel turns underneath, you can feel the join between it and the lip.
Also look at the swage line by the rear arch. There is a slight curve outwards of the panel. If the repair is a plated repair (welding metal on top of rusty metal), its all smoothed out with wads of filler, and the wheel arch lip will noticeably widen, compared to an unrepaired side, and the curvature of the panel is exagerrated.
£3k covers a car that will pass a MOT. That covers a wide range of cars. If anything, its a tricky price point, because top bodged cars can be priced at that, as well as very honest cars. A £3k Mk1 from a dealer would, I suspect, be pretty rough with lots of “good condition for age”. The newest Mk1 is 22-23 years after all.
Think how much welding a 1980 MG or Sprifire would have had by 2000. A fair bit.
I wouldn’t not be put off by a car with sill repairs, if either it has some documentated evidence of a repair, or is cheap and has highly visible rough and ready repairs. Otherwise I would assume its mostly filler, and I should put aside around £800 for arch and sill repairs. Someone describing a car as “rust free” is either a liar or is clueless. The cars rust from te inside out.