Ignoring all the unhelpful replies above. What country are you in? If you are in the UK I'd say it isn't worth the cost and hassle. And as others have alluded too, it's not as if a Series Landy is draft free anyhow, and you do have the large vents below the windscreen.
If you are in hotter climes, then air con might be a more sensible choice. Essentially it's perfectly possible to fit it, either a complete custom setup, or using bits from other Landy's (Td5 Defenders could have air con).
You'll need to make room for the condenser and you'll need a compressor and some way of driving it. Plus the plumbing.
If you have a stock Series engine, then air con will likely sap a lot of power and ruin the already poor mpg. Even Tdi's don't like running air con without a serious affect on performance. So you may need to consider bigger plan to truly make this work.