Sports shouldn't fetch that much of a premium over an SE, as they were the same cash new (until recently) - Dave R will discuss this at length if you let him
S-Line's do fetch a premium, but they were more cash new. Retrofitting the bumpers, suspension, interior etc is very unlikely to work out a cheaper option, so you're best to buy the best you can.
An 06 plate S-Line will generally go for more than £11K, so to do that, you are going to need to look at one with a few miles on the clock. 20-25k a year on a diesel isn't unusual though, lots of people run these as company cars putting motorway miles on them, which isn't particularly hard on the suspension, transmission, etc.
If you're going to be doing low to average mileage, that'll bring the mileage closer to average. If you're going to put 20k+ a year on it, then in 3 year it'll have 140k on the clock, and will be a pig to sell, so bear that in mind.