I usually do the grounddriving starting in a bit...but! I do lots of lungeing with the bitting rig(which means here, saddle/backstrap/crupper and bridle and bit...by the time they are ready for ground driving they may also be in breeching and breastcollar/traces too). They should be very used to carrying the bit before I ground drive. I will also attach a line or rope to the bit, and start teaching them to 'give to the pressure'--turning head, slowing down, etc. The lungeing also teaches them about whips--that they are used for cues and give emphasis to your commands. I desensitize them with the lunge whip, running it all over, waving it over, around, and under, under they are standing quietly and with no fear. On some, I will use the whip to move them over, tapping on the side, like I would ground driving(only from the head, not the butt).
Depending on the horse, I usually just stand just to the side(and just behind) the hindquarters, out of firing range, but effective with my reins. Because the horse has enough to learn during this stage, for the first couple of drives I don't actually cue with the whip, but I do hold it, just in case. If the horse seems like it might try to whirl and face you(many do), have a person at the head, walking alongside until the horse relaxes. Have the helper step away until you are the only one there.
Most horses pick up the rein signals very quick. What creates hard mouths is people hauling or using a steady pull--the horse soons learns to brace himself and will not hear anything. Use a gentle bump(and if the horse doesn't hear you, you can make it stronger), and give just a bit with the outside rein to turn, and usually the horse will figure it out. Stopping is much easier if you do have a solid whoa, as you use the reins in conjunction with the voice and learn it. From a walk, even a horse that has no clue what whoa means is easy to stop, and you don't have to be hard or jerky about it either. Keep asking until they give it to you.
However, since you are starting Mingus without prior training to drive experience, using a halter and two lines is probably better, that way you can make mistakes and it won't be as sharp as with a bit.
Before ground driving(with a bit, anyway), I would suggest longeing him in one first(and yes you should use a lunge line...sorry Mingus, suck it up!). That way you can watch him and figure out what he likes. The basic starter bits are usually a half-cheek snaffle, a half-cheek French Link(if you can find one), or a mullen. The reason I suggest half-cheek is so that it is less likely you will pull the bit through his mouth should something happen. Some horses don't like snaffles(pinches or they have shallow mouths or don't like the movement), some don't like mullens, etc. I would probably get him a basic no-pinch curved snaffle.
I measure with a pencil, mark where the other side of the mouth ends, and measure that length. Go a size larger if you aren't sure.
I would start him out in an open bridle(duh, what did you expect from me?)...with a noseband(not tight, he should figure out where his mouth should stay by himself).
As for surcingles...you can just use the harness saddle. Buckle any loose straps and away you go! Surcingles can be helpful...but usually only if you know what you are doing and what you want to get from the horse.