Not sure the intentions but if wanting a quad good on the track & trails / all around I would pic the canam over the Ltr mainly cause of width & how well the Canam handles in stock trim. Not to mention the price out the door.
As far as handling The canam in stock trim has been the best 450 IMO. Great suspension & feels so light in the air, like a Canadain 250r? lol There was actually plans to make it a 2 stroke from what I read. For how high it sits up it is very stable on the track, unlike some other 450 that you feel you need to widen them out asap.
Mods: The ds450 out of the box has good top end & not much low end, but Put a good full system do the wire mod, maybe an intake & she will be ready to go. It will hang with or beat almost any 450 that has the same mods or a few more mods, like cam etc.... well maybe except the ktm. I don't think it will be an issue for your Dad, but after that it is hard to gain much more hp.
Overall it seems to be a reliable Machine, check the frame bolts, change oil, check oil after running like manual says etc.. & yes invest in a good torx set. lol
Maintenance even getting to do the wire mod makes you wonder if they were worried about some one stealing that wire, cause it is a bit involved & takes a bit to get use to how the machine is put together. Yes & if you want to do motor mods you will be removing the motor. Yet I know plenty of 450 which make you wonder if the engineers thought you should be able to access the carb?
I am not saying the Ltr is any slouch or any 450 is. I don't have have allot of personal experience with them. The Ltr is already wide if thats what you want. Handles well & Mods well. Good thing is both of your choices are honda bolt pattern so tire swaps are cheaper if you already have a Honda.