high emissions - possible reasons?

Will have a look at a Rover 75 that is for sale for very small money in my area, equipped with the 1.8 K-series, 113k mls.
It failed MOT for high emissions, allegedly the seller had it checked by a mechanic:
"Its a problem with valves Got a rover specalist to look at it last week he said hed fix it for 450 . He said that we could fix it our selves jus takin out head gasket and stuff could be tricky for someone who doesnt kno much about this area.."
Sounds a little weird.
So what do you think could be the main reasons for high emissions, what would you check?
Any possible faults an OBD reader would detect?
Or is it a 'stay away!' ?

I've had cars in the past fail mot on emissions for blocked or damaged cats , blowing or rust holed exhaust systems ,lambda sensors and an air control meter.
Is the engine management light on??
If it is plug a obd2 reader into it a get the fault codes

Hope this helps

Obd only good for mems3 engines. Unless you have a T300. Unless you can spot anything obvious it could be a lot of things from worn engine valves as stated to catalytic converter; manky airfilter, dodgy plugs etc etc.
Actual gas readings might give a clue.:shrug: