clydsdales are known as 'heavy horses' which were used in the old times for ploughing and carry coal from the mines.

Of course they will be very dificult to ride as they arent really riding horses.
They are normally larger than 17hh your average riding horse being 15hh-16hh

They wont necassiarily (sp) need a larger feed than other horses because the average food amount for my horse is 11.5kg per week with him being 15.2hh, and for how much taller the horse is you add 1kg every 2inches (as it is 2inches to every hand)
overall per week it costs me about £20 for feed and hay and shavings. As you can get 20kg worth of feed for about £7.00

You will need more straw/shavings/hay for heavier horses so they arent uncomfortable when stabled.

As for cost, its hard to say an exact cost depending on the seller chick.

I hope this helps you