CT and MT restricts Combat Armor and Battle Dress to Military and "Elite" Mercenary Units. That is the only restriction I am aware of. Elite is such an overused term. How do you define Elite? Considering that in a great many cases you need more than just a Mopp Suit, Do Non-Elite Mercenaries wear Vacc Suits, or only fight in friendly environments? So in that case an Elite Mercenary unit is one that is equipped to fight in all environments.
IMTU, for a unit to be equipped with Combat Armor and work within the Imperium, they must hold an Imperial Charter and their licenses must be in good standing. Battle dress is a little tougher license, generally long serving Mercenary Units only. You need a similar license to procure military grade armored vehicles as well. Sure you can, with minimal licensing buy ATV's or the G-Carrier from LBB3 (Not the Striker one which is milspec.) choose from the Paramilitary line of vehicles all day long. Nobody will give you a second look. Military grade armored vehicles, you better have your licenses in order.
We are talking about spending several million credits to equip a platoon. That in and of itself is an elite unit.
The US hasn't had an ATGM with a range of under 2500m since the Dragon. 2500m is the assumed effective range of a well trained crew in a T80. The Tow II was and still is a ground launched ATGM. You can launch from a tripod, a specialized vehicle, a HMMWV, a Bradley, or an attack helicopter. On the Helicopter mount it carries 3750m or wire, all of the ground mounts carry 3000m of wire. Javelin range is 2500m. Hellfire is 6000m. Those are the US Army's current inventory of ATGM. (Mavericks belong to the Airforce, though I have seen a picture of a pair mounted on an Apache.)
Yes you can dig in an armored vehicle. (Though that does require specialized equipment, of more than a shovel. While some tanks have dozer blades, I have yet to see an IFV with one.) Infantry, with strictly organic assets, (IE entrenching tools.) generally have shallow positions within an hour. (Which will keep most artillery fragments from doing anything.) 4-6 hours full, positions with overhead cover and concealment. (All but a direct hit stopped and frontal hits by most tanks absorbed.) Terrain, of course, dependent. Infantry positions are much more difficult to see than tank positions as well.
But again you are missing the point. I am not, nor have I ever advocated, a non-combined arms approach. I am claiming that at about TL12-Tl13 while the tank has a role the APC no longer does. And further since the majority tickets available to Mercenary units are going to be, by the very nature of the Imperium, in hostile environments where vehicles have no room to maneuver, and the collateral damage caused by their weapons is catastrophic, you are going to have to look at lighter forces. Combine that with the Price on all of the Canon tickets, and not only are armored vehicles going to be the exception, not the rule, even when you can use them, but a Mercenary unit can't afford the capital outlay to buy them in the first place and still equip their infantry.