Restarting DirectTV with previous equipment

Also, do they offer a first responder discount like ATT cellular/firstnet service?

Nope, DTV will not reactivate used receivers and even if they did you would still pay the sames fees.   DTV has no group discounts.

If the equipment that you have is Current HDDVR or GENIE or Minis -NO SD Receivers will be activated as they stopped support mpeg 2 a while ago. They may reactivate them and as soon as they do the $7 a monthly fee starts again. 

Depending on how many TV's (ROOMS) you require - I suggest just going for a HS-17 and adding wired Geminis that may help you keep the monthly cost down for receivers as the HS-17 is a Head unit only as does not connect direct to any TV. you will not be charged a $7 for the HS-17.

Signing up with your current receivers or with different Equipment will start a 2 year commitment anyway.

They have to add a mini client to replace the HS17 so they still have the same number of TVs so there is no savings.

If it hasn't been that long since you left DirecTV, they might reactivate the equipment along with it depending on the models. Though they don't allow reactivation of individual boxes anymore, doing it at the account level may be different (such as reactivating out of non-pay disconnect).

Reactivating an account doesn't come with a 24 month service agreement unless you get a winback special that comes with one. If you just turn back on with no promotions than it is still month to month. Haven't heard of that changing.

The additional TV fees and receivers services do not go away. As long as you have one DVR, you pay for DVR service. Simple as that.

There is no first responder discount as unlike a phone, you're not using your TV to keep in critical communication. Pay TV is luxury entertainment, nothing more.