Do you have a p/x to go with? If so get the best price possible for that, see what glass's valuation says which is what trade use i believe. That should reduce the difference you pay without them reducing price of car. They are usually more flexible on pricing of the extras than the price of car.
If you go in with a serious offer and have funds available you should be in a strong position they shouldn't dismiss a serious offer just stick to your guns. I find it helps if you have other cars in the offing but have a good reason to choose that one over the others, ideally other than its a good price!!
Dont be afraid to haggle, do bear in mind that there is a bottom line though and they wont make a loss on it even if they need to shift it.
It sounds a great car at a good price just ask yourself how much you think its worth and if anything else available comes close. Besides you may even get the extra back if/when you come to sell. I have a feeling you wont regret it whatever you pay 12-13k.
As for the warranty also consider your plans for the car, if your on this forum you will soon get the modifying bug!!