The best upgrades that they could have are already on them, smoke launchers. Pop all your troop choices in them then do a blitz rush with them and the launchers, disembark, and harass. Seriously, that is what they are good at, that is all you really need to do with them. Save the points because those extra 10-30 points could easily buy you some upgrades somewhere else that they would be more useful.
As for what is worth it's points in general: Extra Armour and a Storm Bolter are worth their points. The HKM isnt because it costs 10 more points than a missile launcher and only grants one shot, unless you're playing apocalypse then the infinite range is essentially useless. Dozer Blades belong only in siege and heavy cover situations (WW2s post d-day time in the hedgerows comes to mind). Still, I think more Rhinos is a hundred times better than better equipped Rhinos. Rhinos are one of many things that the Imperium have a surplus of, and as such there is no worry about losing them.