You are quite simply mad. There are so many things wrong with your plan I don't know where to start.

You initially mentioned having a loan for your car and, I assume, to consolidate and overdraft and credit card debt. Already at the age of 20 your spending is out of control vs your earnings. You need to sort that out first, regardless of the car you're driving. You haven’t even moved out so you’re living in the cheapest place ever and you still accrued debt? And don’t reply with the “I’ve learned from the error of my ways” as that’ll not wash either.

Next is the issue of being accepted in the first place. Do you think a 20 year old with an outstanding loan would be accepted? I doubt it, unless you’re on mega-bucks (you’re not). You appear to have a sound, reliable motor stick with it. If anything you may have over-purchased for that.

Give it up as a bad idea, get your head out of clouds. It’ll not happen.

It sounds like you're interested in the Mercedes SLK 250 BlueEFFICIENCY, you'd be doing well to get 40 mpg on average with normal driving, put your foot down (which you will do) and expect similar fuel consumption to the Focus 2.0, so no fuels savings to be had.

I have one of these now and regularly see over 50mpg on a 52 mile communte each day (40 miles motorway the remainder mainly fast-ish A roads).

Even if I give it some welly it never drops below mid 40's - this is one reason I bought this car. My previous 3 litre diesel Jaguar XF was brilliant but the quoted 44mpg was only ever achieved very occassionally - my average, when I handed the car back after 2 years, was 34mpg. So, a significant improvement in fuel ecomony was achieved by me.

I'm on a PCP and my repayments baed on 20k miles per year are [to me] a reasonable £519 per month. This is exactly the same as I was paying for the Jag and although it was by far the best car I've owned to date the depreciation was simply eyewatering. In my opinion a 3 year old 3 litre diesel Jaguar XF makes a cracking second hand buy. All that car for less than £20k.

These heavily subsidided (by the manufacturere) lease deals are popping up more frequently. We're looking at doing this and selling off the wife's Fiat 500 which has depreciated by about £7,500 over 3 years. Thats £200 a month depreciation. Take that depreciation off the attractive lease rate and thats the extra you're paying to drive a luxury sports coupe.

To the OP if you sell your Focus, pay off the loan. If that means you then can't afford to stump up the lease deposit and a few months payments then you can't afford it so you really shouldn't do it. Save up to be able to afford to do it or save up to one day leave home, start your own business and when you need the start up cash and you've blown a load on a car you'll have to borrow outsider money and share your business with people you wouldn't normally go for a pint with.