1. Don't know.
2. Check rightmove and other letting agencies in the area, it depends entirely on where you want to live. The college is in Whiteley which is a well off area with a lot of family style houses. As a result, you're looking at more to rent/buy there than you would in Southampton or Portsmouth. Bonus being that you don't have to try and get into Whiteley every morning, which is a nightmare.
3. The salary will stay at your starting salary whilst you're at the college, it jumps a bit when you finish the college and get posted to a unit. How long that takes depends on what course you do and how well you do at it. For a tower or approach course it's less than an area rating course. Expect a year at the college and you won't be disappointed.
4. The college course is at Whiteley and then you're posted to a unit. You can express a preference, but you'll go where the company needs you and you could well end up back at Cardiff having just moved all your family down here. Likewise you could end up in Aberdeen.