I dont think its likely youll find one for £10,500... unless youre very lucky and get F&F somehow. Its most likely that youll find a brand new 182 for about £12,000 at the cheapest (once youve accounted for the road tax etc) from a car supermarket. When I looked in January, I couldnt get one for less than that - and even then, that was without the Cup packs. Im not suggesting its impossible for everyone, but for the average Joe its nigh on so.
The trade value of a 3 year old 182 is predicted to be around the £7k mark, so youd lose about £4-5k over that period. I dont think you can argue on financial grounds that a brand new 182, even discounted, is going to lose you less in depreciation than an older one or a 172.
What you can justify a 182 on is the marginal improvements in handling and poke, coupled with the "newness" factor. You will pay a lot for the privilege though. However, most of the benefits will be present with a car thats 1 year old - and thats what Id suggest you thought about, because finding one at £10,500 is going to be a very long slog.