The 'sch' realy differs from both 'sk' and 'sh'. It's a bit of a filthy way but i can't make more clear than to tell you that it resembles the sound that one makes in the throat before spitting in order to clean the throat. Try to make that sound (and leave the spitting behind ;-)) ) and you might 'catch' the dutch 'g'-sound ('sch'= 'sg' here). At first it will sounds much to harsh but gradually you'll be able to the soften the sound to the way the dutch use it. Good Luck!
'een' as 'one' is a number; 'een' as 'a(n)' is an article. Both are possible here so in principle your answer is correct. But 'een' (number) spoken has a long 'ee' sound like in 'way'; 'een' (article) sounds like 'un' like in 'sun'. The latter sound can be heard here so the article is ment, not the number.
I work in a primary school and once an 8-year-old kid came up to me and said 'Miss, I'm afraid I forgot to change my shoe.' I said, 'You mean you forgot to change your SHOES?', and he said, 'No, just one of them. I literally forgot halfway through'. And there he was, wearing one trainer and one winter boot. Nothing will surprise me ever again...