I am not in my 60`s but have shot crossbow for a couple of years . The salesman trying to sell you something you don`t need or want is no good . As stated by others the 308 , 315 or 330 have a ton of energy to pass through a normal animal . I had plenty of passthroughs with my older slow Wolverine doing maybe 250 fps . All speed does is make a slight range miscalculation less of a problem .
For the most part the faster the bow is going the further into the ground on the other side of the deer the arrow will be buried .
Your question about treestand or ground hunting . Your choice . Some individuals as they get a little older don`t feel as secure up at heights as they once did . Animals have been harvested from both .
Good luck with your purchase .