I don't think the trophy wife explanation works at all. The term is a recent coinage in our culture and has nothing to do with the life of nomads on Esos. Certainly Drogo is not going to attend cocktail parties at ritzy country clubs where he will introduce Daenerys to envious colleagues (including many older males). She is not a clever, cultured, and independent woman who can stand in for him at important social functions and perhaps some business meetings. More to the point, I see nothing in Dothraki culture that indicates that an older man would gain status by marrying a young hot foreign woman. Note that this is a marriage, definitely an arrangement even if not exactly a sale. Conquering cities or tribes and carrying off women gains a man status in a warrior culture. Making deals with a fat Pentosi cheesemonger does not.
A better approach to our question would be to consider what a khal would want from a wife (not from a bed slave or a mistress). I think the answer can be given in two words: strong sons. Drogo is not stupid or ignorant. It is an extremely good bet that he knows about dragons and Valyria. He certainly knows about dragonbone and Valyrian steel. A man like him would be very interested in mating with the last female descendant of a guy named "Aegon the Conqueror." The appeal would be based on genetics, what the residents of Esos and Westeros would probably call blood lines.
This explanation holds up well when we consider what happens after the wedding. There is the long trip across the grasslands, the ceremonies, the prophecy, etc. Did Drogo think that the dosh khaleen would be highly impressed with a beautiful platinum blonde? Not likely. The whole business is centered on Dany's pregnancy. You have the mixing of the blood of a mighty khal with that of a descendant of dragon riders. That is the main point. In the chapter where Dany presents Viserys with the Dothraki garb, she reports that her pregnancy has inflamed Drogo's desire, "and of late his embraces left her exhausted." More significantly, we have the ceremony that starts with the eating of the stallion's heart. After Deanerys succeeds, Drogo puts a hand on her arm. It is the only time she senses fear in him. He is uncertain what the crones will prophesy. When they proclaim that the unborn child will be the stallion who mounts the world, it is the fulfillment of his hopes.
I don't deny that the khal is happy to find that Dany is good looking. I don't think that her beauty was the deciding factor for him though.
I like this explanation. I think the dragon eggs were a gift from Illyrio to Drogo not Dany to prove she was a Targ.
If Aegon is a Blackfyre is it possible Illyrio wanted to split Dany & him up before he started producing more Targs. Dany points out they would be married if circumstances were normal and Dany is at the pivotal point where she's just becoming a woman. Illyrio might have wanted to split them up without killing them. Viserys insisted on going and Illyrio backed down as:
1. He's a fool and if he did start shagging Dany someone would find out
2. When the Dothraki did find out he'd be dead