Car is a mk3 1.6 golf.....recently it has started to loose power and today it seems to be really bad I can barely get over 60mph

I will be taking it to the garage, however incase its properly fooked I decided to look for another engine. I managed to find one with 74k on the clock for about 120 quid which i thought was rally reasonable.
My question is how much would i garage cost to take the old one out and put in a new one?
thanks