My view - take the money! I'd probably do that even if I wasn't looking to get on the property ladder.
If you're set on a Dooper, you could always shop around for a cheaper one and bank the difference.
I've just faced a similar dilemma over whether to spend more money on a more recent model or less on an older one (never considered an oil burner, though - just can't bring myself to buy a diesel). The important thing is not the age but the overall condition of the car and care it received from the previous owner. OK, an older petrol model will be slightly more expensive to run than a newer model, but I guess it depends how long you're going to keep it.
In the end, I went for a 2003 Cooper S (1 owner, 47k miles) for £5700. On older models, there's more choice but I'd steer clear of heavy mods, very high mileages, multiple owners or models without FSH.