First - You don't need to modify a standard Defender to take it off road, period. Probably not a good idea while under warranty anyway. You certainly don't need a lift (ever) and IMHO bigger tyres are a hindrance rather than a help. You'll be reminded of this when you get a puncture and need to lift the extremely heavy, wet and dirty spare off the back door, never mind the increased turning circle everyday. If it needed wider tyres LR would have fitted them 20 years ago. Oh,  they already did. Went from 7.50 to 235's. :) I'm very happy with the original Continentals and would buy again when it comes time to renew. 
Second - Don't buy any accessories from the Dealer. You pay an extremely high price including extra tax for the convenience and will do much better aftermarket if you really must add some baubles.
The only extras you really need are a towbar and front mudlaps. A stereo upgrade with bluetooth is also worthwhile. (To add some bass you WILL need to fit a subwoofer.) Also, throw some sound deadening matting in the back and you're done.
See here for my experience with mudlaps   
http://www.aulro.com/afvb/90-110-130...electrics.html   Try Karcraft in Sydney for pricing.