Looking online for a T25 and one I am interested in has these advisories on the MOT:
Monitor and repair if necessary (advisories):
Integral body structure is corroded but structural rigidity is not significantly reduced BOTH INNER WHEEL ARCHES (6.1.1 (c) (i))
Nearside Front Integral body structure is modified but structural rigidity is not significantly reduced WELDED FROM OUT SIDE CORROSION TO INSIDEOF STEP (6.1.1 (d) (i))
Offside Front Integral body structure is modified but structural rigidity is not significantly reduced WELDED FROM OUTSIDE CORROSION TO STEP INSIDE (6.1.1 (d) (i))
How bad does this sound and what do you think it would cost me to have it properly corrected if I bought it? (I can't do it myself)
Depends how far the tin worm has travelled and how much needs cutting out.
Ask the seller for some pics. Repair panels are readily available. Then take the pics to a body shop or two and get an idea. Factor in cutting, welding and a bit of spraying.
Westfalia Hightop Top Joker 1.6TD - aka Daisy, my labour of love