I started with a 400/450in early 80's kept my single stage presses for a couple of years upgraded to 550, got rid of all single stages and turrets replaced them with bonanza coax. I like having the simple 550 and being able to load everything else except the main comp pistol/rev cowboy cartridge that i and wife use several thousand per month on the 650 which is always set for. The bonanza coax is dragged to range for load development. glad i kept 550 after buying 650(two months ago)will change from 38spc to 45acp and 40sw after i get stock built up. After a head injury multitasking was a challenge but what i really lost was confidance in not messing up on the 550. Even modified one 550 toolhead to add a powder check and loading previously sized and primed cases. the auto indexer made all the difference and reloading is fun again and i feel safer.
Back to the original question, I would keep the 550 load 223 and 308 ,9mm. The 650 shines in large quanity pistol. for now the 550 will do and if he is just engaged he really does know what his goals may be and priorities will change. You may not be able to afford a 650 if you family expands