Last week I bought a cheap ford focus with 12 months MOT. Works nice. On closer inspection it has a lot of rust. Is this legit to pass mot?
How much do you think to repair? 
Is the underneath subframe and suspension arms too rusted?? Thanks 

That not rust it's just Fe₂O₃

Is this your first car, or the first time you've had a look under a 16 year old car ?

hope it was really cheap 😮

Even for a Ford, that's pretty crusty!   Was it bought from a seaside area?

None of that would fail MOT yet. 

To cut out, plate and paint both front sill ends and the rear arch would easily be over £500 at a bodyshop.  Not worth the cost.  It'll be worse underneath the plastic sill covers as well.  MOT testers aren't allowed to remove the covers.

Those rear wishbone bolts aren't coming out.  Best just to leave them and then swap the whole rear axle as one lump if anything there needs replacing.

I suggest just live with it, may have been caught a few years back, but I think this one is beyond economic repair - you could always make a feature of it like me...