Congrats on the find!
If you've been researching then you probably know this already... be very gentle with the top lid. The hinges have a tendency to weaken. It's the same with the front door around the light. If the Rocketeer had any weak points, those are the two. They make replacements for the fold over spool pins, so that's not an issue. And- if you happen upon a Singer 603e that's being sold at a crazy/silly low price... then you buy it because the motor is identical to the one in the 500a and it could be a big cost saving measure to have a spare lying about. This is not to say that the motors are weak, because they are not... but we are talking about a fifty something year old machine.