I think it is really up to you and how you make the transition when you are training
You start with an open bridle til the mule gets the idea of what you aer asking , then you move on to the same work with a blinker bridle til he gets it again . . .
Horses and mule of the past got TIRED from a full day of real WORK. They were GRATEFUL to stop and stand quietly when they got a chance. Draft animals were worked at slower paces, mostly walking, looking at familar places on the farm, not scary things. All very "routine".
If you go up to the sticky notes at the beginning of the Driving Forum, there are a lot of posts on using blinkers. Most all equines are started open bridle during training, then go into blinkers when attached to a vehicle. Blinkers focus the animal forward, he doesn't see partial things happening around him to react to. Equines QUICKLY learn the routine for hitching, driver climbing in and want to help, SAVE you the work of touching him with whip or even needing to ASK "walk-on"! This can get animal moving before you are prepared, not in total control.
Not sure if you are familiar with training mules, but they NEED to totally understand what you want, before they will try for you. They want to succeed, so all the details have to be quite clear to the animal, he needs to know you won't let him get in trouble. If you haven't trained a mule or donkey before, you will want to check in with the American Donkey and Mule Association. Their information is very good, will help you be successful with your mule in the easiest way possible.
Get measurements of your animal, so you can measure harness at the sale. Most folks start at the girth measurement. A wider backpad for a single harness is easier on his back. Padding should be solid, can't feel the screw ends of rein rings (terrets) thru it. That backpad should have a frame/tree inside, so it is stiff, keeps any weight off his spine with 2-wheel cart shaft weight. No frame means it is for a 4-wheel vehicle, which is not what you want to start driving with. Wider breastcollar, V shaped allows room for the windpipe when he puts his head down to pull. Measure from lip corner, over the head to other lip corner, for length needed in bridle. I would buy a bridle with blinkers, square, not the cut-away type on draft bridles. Square will be easier to center his eye on, not allow any peeking under or around. You want a full noseband, keeps the bridle cheeks closer to his face when you work the reins. Again, no peeking under the blinkers. You want a nice wider breeching for around his haunches, spreads out any weight when he stops the vehicle or holds it back on down hills.
Leather should all be soft and flexible, twistable without ripping. I would avoid rusty buckles, thin, BENDABLE buckle tongues, thin leather straps that are stretchy or worn. All are places that will give/break when stressed in work. Reins should buckle to bit, not snap. This buckling on each use, keeps you checking the leather for wear, might need new billets. Snaps break easily, and no one EVER seems to check the leather where they fold for wear. ALWAYS are surprised when a rein breaks!! Get reins that are comfortable for YOUR size hands. Wide and thick are tiring to use over a long time, just don't fit.