Andrew, cracking deal on the merc. Nothing wrong with buying an older model, I worried a little when I got the 997 but actually it really doesn't matter as 90% of people on the road won't even notice.
The deprecation on the merc will be a little heavier but you are going into it knowing that and as you have paid no deposit it's not the end of the world. If you went for a lease you would have to pay to get out of it so which ever way you buy it there will be a cost but I reckon the pcp will be cheaper.
Looking forward to pics of the sls and the new merc