Sean and BHK9 has pretty much put out all the options you need to consider before going for any of your choices from the House of Lange. Having had the fortunate chance to handle all three closely/intimately, for me at least it boils down to your lifestyle. They all offer the same range of versatility (dress up/down etc) one can expect from Lange, quality/finishing/design is nice enough to make some of the Swiss watch houses shake in fear of what Lange will come out with next.
Personally, based on your choices above, I will take Dato as my first Lange. Why? It is the quintessential Lange. Like a BMW or a Merc. Totally tuned, stoic and unrelenting in relaying its usefullness to the user with its butter-smooth pushers and that date which is so useful. Why not the Perp? Well I think if you get the perp, it will be something that is offered by almost all the other watch brand out there. The layout isnt anything extraordinary that shouts out Lange BUT that is saved by the finish by which Lange is made for, plus is made in a classically nice size of 38mm. An added bonus of owning a Lange is winding it when power reserve gets down. You will miss this with the Perp and also, sure it is practical with all that data but do you always want to own safe/practical choices especially as an introduction to Lange?
Whichever you choose as your first Lange among the three, the one sure thing for me to get as my 2nd will be the DS. It is like an M5/M6 or a Lamborghini Gallardo. I mean, the thrill of one's journey into something IMHO is enhanced further by the progression of levels of tactile sensory sensations. What thrill is there when one can just opt for the supreme level of anything? 
Have fun choosing and kindly share wristshots with your decision!!! 