So throwing my hat into the ring but the most likely at least for now even with some having a heavy cost are plausible.
CR90 Corvette
Marauder-Class Assault Corvette
IR-3F-Class Light Frigate
So for the IR-3F I know its nothing all that impressive but the cheap cost good speed and its defined role would make it tempting for the mc since he is planning to prospect other planets in system and wants more foot traffic coming to him. Shielded, decently fast, meant to monitor the system and overall a rather simple craft meant to just have presence throughout the system. Would be useful for when he starts getting more people as it could be used for inspections and to try to tag any malcontents before they leave system. Not meant to be much of anything but good for its role.
Maruder-Class Assult Corvette. This one is far more expensive but is known to be a solid craft, hyper space capable decent armaments along with the possibility of a Diamond boron missle refit to smite those pesky starfighters ( boron misiles hard counter starfighters weapons and are a rather nifty little miss with a decent punch as well) For this craft i think new runs around lile 3 mil credits rather hefty but it's meant more for deterrence and for those who come to cause trouble.
CR90 Corvette
With the mc wanting some economic boom these are a solid craft with decent tonnage capacity and can even carry a tiny escort with them so not to bad. Overall a solid craft.
Notable thoughts for others:
This was not listed but man is it a nice overall cheap craft that is pretty customizable
https://starwars.fandom.com/wiki/Gozanti-class_cruiser . Something to consider since they can be refit in a number of ways.
Other options:
I can see him getting that sweet Lancee class but only after he proves himself and maybe leans on his family to get one or one on the cheap. He'd have to prove himself competent and his system worth the investment but he is kinda already leaning towards that direction anyways and having a star fighter killer lime that would be good for him.
Realistically though that is alot of credits to drop on one ship so it would be quite awhile till he could get it as thats not the kind of ship you just wonder into and get. Alot of the options are great but unless you well want to handwave the issue of where he is getting the capital or leverage to get some of those other ships they really won't fall into his hands otherwise. Especially since he is focused on the system itself and seems to be determined to try to keep the mega corps out. So he won't be raking in as much credits in the beginning if he dealt with mega corps instead of the smaller in system businesses. Long term he will have the option to buy or get some of them but as for now realistically unless he strikes "gold" in his system its just a steady build up overall.