Quite simply, the statement was true from Obi-Wan's point of view, with no agenda needed.
In fact, if we add two words for context, the statement becomes 100% accurate. Obi-Wan should have said "Your father would have wanted you to have this when you were old enough." BOOM.
Although technically a lie, there wasn't necessarily any agenda on the part of Obi-Wan in telling Luke this. We know from the Disney canon that Obi-Wan was finally telling the truth in Episode VI - as far as he was concerned, his friend Anakin Skywalker "died" when he became evil. The actions & motivation of Darth Vader were different enough from his former life that he could reasonably be considered a different person altogether.
LUKE: You told me Vader betrayed and murdered my father.
BEN: Your father was seduced by the dark side of the Force. He ceased
to be Anakin Skywalker and became Darth Vader. When that happened,
the good man who was your father was destroyed. So what I have told
you was true... from a certain point of view.
LUKE: (turning away, derisive) A certain point of view!
BEN: Luke, you're going to find that many of the truths we cling to
depend greatly on our own point of view.
(Ben sits down on a nearby log)
When I first knew him, your father was already a great pilot. But I was
amazed how strongly the Force was with him. I took it upon myself to
train him as a Jedi. I thought that I could instruct him just as well
as Yoda. I was wrong. My pride has had terrible consequences for the
LUKE: There's still good in him.
BEN: I also thought he could be turned back to the good side. It couldn't be
done. He is more machine now than man. Twisted and evil.
From Obi-Wan's point of view, he knows that the good man who was Anakin Skywalker would have wanted his son to have his old lightsaber. Had Anakin actually died during the Clone Wars, the same could be said regardless of whether or not Anakin actually said at some point "I want my son to have my lightsaber". Having been his friend for years, Obi-Wan would be able to make a fairly accurate guess about Anakin Skywalker's wishes on the matter.
As for the statement being a lie, that is technically true, but it's really no different in context than a parent handing down a family heirloom saying, "Your grandfather would have wanted you to have this." There is no reason to assume that Obi-Wan intentionally lied just to recruit Luke, or that the specific request occurred off-screen. He is simply telling Luke what he believes to be the truth - that Anakin Skywalker would have wanted Luke to have the weapon.
The bit about his uncle not wanting him to have the weapon would also be true without an actual statement to that effect. Owen Lars had made it plain that he wanted Luke to have nothing to do with the galaxy at large or even Obi-Wan himself. Common sense dictates that he wouldn't allow a teenage Luke to have such a weapon, and in fact would probably just throw it away if given the opportunity.
Note: As many have pointed out since Episode I, yes, there are obvious gaps in what Obi-Wan said on Dagobah. Nothing he said is untrue, however. He DID take it upon himself to train Anakin whether the council approved or not, and he DID think he could train the boy as well as Yoda, who personally disapproved of training Anakin at all.
He was also shocked to learn Anakin's midicholorian count, and therefore how strong Anakin was in the Force. Also, ironically, we now know that it was from Anakin himself that the morally rigid Obi-Wan learned to be "flexible" with the truth in such a manner.