As has been said, 1st build your weapons to be removable/interchangeable.
Magnets work well for this.
As does simply bitz ordering via eBay etc.
Next up? Whatever you envision the rhino doing, add that Havoc Launcher.
It's free
atm & there's never a time when having more dakka available is the wrong answer.....
Later on, if it should cost more pts than you want to spend? No problem as you've followed our advice & modeled it to be removable.
As for what type of combi-weapon is best?
Only you can determine that.
•do you want more dakka for plinking away at infantry? Pick the combi-bolter or flamer.
•do you think you'll need the extra
ap of plasma?
•Or pick the melta & see if you can go vehicle hunting.
Wich you should take really depends upon what opponents you expect to play.