Every vehicle in the Imperium can be outfitted to better suite their role during certain missions or on planets with terrain difficult to traverse through. Vehicles of the Ordo Malleus are no exception to this and they have access to many upgrades other factions don't. An Inquisitor can spend some of his available resources to obtain and equip special components on his vehicles and Dreadnoughts when he finds it fitting.
Daemonhunters have access to many upgrades simply because most of them don't come default on vehicles, which newer codices have the luxury of not worrying about as their vehicles usually have these for free. As usual, I'll list the upgrade, the points cost, a brief description, and my personal opinion on the upgrade in italics.

• Blessed (10 pts.) - This Grey Knight vehicles have been consecrated thoroughly and is equipped with a pure machine spirit. The vehicle gets a +1 bonus to armor penetration against enemy vehicles that have daemonic possession. Dreadnoughtswith this upgrade also have the Aegis special rule with Leadership 10.
  
  One of those only useful is certain situation weapons. This also references several other upgrades, but I believe they no longer exist and can be ignored. It effectively boosts the vehicles Strength by 1 against Defilers and the occasional vehicle with this upgrade. It also gives the vehicle a personal psychic defense, but that won't really matter as you will probably have a psychic hood. Overall, a decent upgrade, but it is so limited in targets that I never buy it.
  
  
• Dozer Blades (5 pts.) - Vehicles with this can re-roll failed difficult terrain tests if they moves 6" or less.
  
  Kind of a pain that this is only effective with up to 6" of movement, but this can be a point dump if you have nothing left. I had it save my Land Raider the one time I purchased it, but I've never regretted not buying it.
  
• Extra Armor (5 pts.) - Vehicles with this that suffer 'crew stunned' are instead afflicted with 'crew shaken'.
  
  For 5 points, this allows your transport to keep moving unless immobilized, which is quite helpful. I recommend spending the points if you plan on using any transport that will be moving around the battlefield. You still can't shoot if shaken or stunned, however, so you should be aware of that.
  
  
• Hunter-Killer Missile (15 pts.) - The vehicle with this upgrade may fire a Strength 8 AP 3 missile with unlimited range once per game.
  
  For 15 points, you get an extra Krak missile shot. Worth it? It could be. But unless taken on several vehicles, I don't find the one extra shot to be worth the point cost. I would have to toy around with it more, but right now I don't use them.
  
  
• Pintle-mounted Storm Bolter (10 pts.) - Adds an extra storm bolter defensive weapon pintle-mounted
  
  Two extra Strength 4 AP 5 shots could be nice for 10 points, but my vehicles are almost always targeting other vehicles, and those shots do absolutely nothing. Maybe it would be worth it with an anti-infantry vehicle, but for the most part I ignore this upgrade.
  
  
• Sacred Hull (15 pts.) - A vehicle with this upgrade is coated with purity seals and scriptures so holy that any Daemon assaulting the vehicle must pass a difficult terrain test to reach it and suffer -1 to Leadership when in base-contact or are tank shocked by the vehicle.
  
  Another anti-Daemon upgrade. A decent upgrade if it affected all units, but for the cost and its limited use, it gets completely ignored. Same as Blessed, I would never buy it.
  
  
• Searchlight (1 pt) - Illuminates a target during Night Fighting, allowing it to be seen by all friendly units firing after this one, but only if it is spotted by this vehicle. Also causes all who attack this vehicle next turn to ignore Night Fighting rules, however.
  
  A one point drop that is a bit nerfed in this codex. As the target has to be spotted, it is a bit worse than many searchlight variants, but it still does something. If you have no other one point drops to use, you could look here. Otherwise, Night Fighting is too rare for this to get use.
  
  
• Smoke Launchers (3 pts.) - Once per game, after moving, a vehicle may trigger smoke launchers. This turn, the vehicle may not fire but any penetrating hits in the next shooting phase count as glances instead. Smoke launchers may be moved even if stunned or shaken.
  
  For three points, you can have you vehicle have an extra defense beyond cover saves. Putting this on a Land Raider or Dreadnought can really increase survivability for one turn, but you can't shoot at all, so I save it for desperate turns or when my Land Raider wouldn't have any nice target to shoot at anyways, or even when shaken. A decent upgrade for 3 points that I usually take on at least my Land Raider, but not so often on my Dreadnought.