a .300 might be a little stiff for deer if you can't find a light load. There is truth to that. However, if you shoot a light enough bullet, it isn't as much of an overkill as folks might think.
My buddy bought a .300 for the reason that he can load it light for deer, and not have to buy another rifle necessarily for bigger game since he now lives in Alaska.
We took our rifles to South Dakota for deer a couple years back, and the .300 was just fine. He shot 150 or 155 grain Federal High Shocks (sorry if i don't have the name dead on) it was just fine for deer. Not much wasted meat.
I would ask myself how much hunting am I gong to do this season before I buy something new. You might be able to hunt dads gun this year, save up more from now till next year, and really get a shooter.

Nothing wrong with anyof the others mentioned here either. For that matter, many of them are probably better for deer. If you are lookng to maybe hunt this season though with your dads rifle, then just find acouple boxesof lighter weight bullets. A couple boxes of these will certainly cost less for this year than a new rifle, and all the godies that go with it.