I know this may be another 'How long is a piece of string' type question but
I thought I'd try to get a rough idea here before asking carpenters to come
around.

I have a 3 bed terraced house with sloping floors - about 3.5 inches over
roughly14 feet (the width of the house). My cousin who knows more than I
about these things says it should cost no more than £600 for a carpenter to
take the old floorboards up and put new, level ones down. I was expecting
much more but admit to being quite clueless wrt diy/houses etc.

Is £600 roughly in the ball park?

Also, is there any risk of incurring damage to the downstairs artexed
ceilings if I get a carpenter in to do this job? Money is real tight and I
dont want to solve a problem upstairs only to face new ones downstairs.

Help much appreciated.