Re: Ford's age, I'm expecting a lot of vehicular chases, and not too much in the way of fisticuffs. I think he'll be convincing enough, and he loves playing Indy (far more than any other role), so he'll give it everything he can.
As far as Disney's plans go, if they think there's going to be much of an audience for a non-Indy spin-off... they're taking a gamble. RAIDERS is among my five favorite films, I love the series. But I'm not watching spinoffs. THE MUMMY, THE SHADOW, THE PHANTOM, THE ROCKETEER, and things of that nature already covered the "counterfeit INDY but still pretty fun!" thing; we don't need to see that happen in the official continuity.