[Christ]

Shifting gears isn't a concern at all.

Everything you need is already there except a lever. Make a lever that travels forward/backward, place it in the cabin where you want it, call Motion Pro and have them make a cable the way you need it that is capable of push/pull motion, then make the mounting tabs for each end of the cable. Remove the 10mm nut from the transmission shift drum actuator arm (circle that clamps on the splined rod with a tab on it), drill the spherical joint out and bolt the transmission end of the cable on it, then reattach it to the trans.
Attach the other end of the cable to your shifter lever and make sure it functions properly, then add stops so you can't over-shift, which can damage shift forks.

If you don't want to use a cable, just make sure you can line up the lever with the actual shift mechanism within a few degrees and then you can use the original setup with a longer rod. You can actually use the original foot lever with a longer rod and place it next to the seat, too. You just pull the bolt in the center of the rod and pull it off, cut the rod in half and weld in an extension.

Braking system will depend on how you set up the suspension/steering and what type of wheels you use.

If you want to avoid brake drag, you can get both solid spoke and wire-spoke wheels with drum brakes in them, saving you the weight and expense of adding spindles, bearings, hubs, etc.

You can mount car tires on motorcycle rims with no problems, but most of the wire-spoke wheels use tubes, so you would need to get a tube that matches the tire size you intend to run.

If you want to use pre-existing parts for the steering/suspension, consider using a-arms from ATVs or UTVs, or if you'd rather, use the entire hub/steering assembly from any car and build your forward frame section around it (You can even use McPherson parts.)