This does confused some of my opponents...
Chaos rhinos can take One Combi-Bolter, One Combi Weapon and One Havoc Launcher.
They also get smoke launchers - which can be used only once, but can be used if you advance.
This seems like a lot of weapons, and the sprue doesn't come with half of them! You will notice there are some weird things in the sprue that isn't on the wargear list... things like sirens/speakers, lights etc. I think they were wargear options in the past.
I generally take Rhinos with a combi-bolter, combi-flamer and havoc launcher (as the launcher is fairly cheap - 6point?) in a World Eaters army when I have a few points left.
Putting something more powerful than a combi-flamer on it (plasma or melta) will mean it will draw too much attention. You want your Rhino to be a low threat target (and people usually consider it that once you've dropped off 10 berserkers) so it can cause trouble elsewhere Killing an empty rhino with say 3-4 lascannons is wasting points.
The flamer is helpful on the advance as it's an assault weapon, and once you've dropped off your troops you'll more than likely be using your Rhino to soak up overwatch, tie up something shooty in melee, getting through the backline for the extra
VP or sitting on an objective.
Also Rhinos are fairly large so you can do some careful and cunning pile in shenanigans to tie up extra troops in melee, but remember that no part of the the rhino can move more than 3 inches and must end their pile in closer to the nearest model.
Also
WYSIWYG is not part of the official rules (may be local/house rules). You could model NO weapons on them at all and claim there are two combi-bolters and a havoc. That's plenty of dakka.
Also check out some 3rd party supplier for "light vehicle" guns... something like Kromlech maybe or Spellcrow.
I think these might work
https://www.spellcrow.com/pintle-mounted-turret-p-508.html or
https://bitsofwar.com/home/936-legionary-tank-twin-thunder-gun.html