You have plenty of room to mount a battery in that frame in the middle. Your bike has disk brakes, which is a requirement IMHO to stop reliably in the rain.
Mid drives are required if you gain 1000' in height in an hour on your trips. If not, geared hub motors will work fine. Geared hub motors require less machine work, and use less chains. They also can be ridden home if you break a chain. They are less expensive.
DD hub motors work, are inexpensive, but accelerate slowly and produce most power in the 20-28 mph range. I personally think an unsuspended bike is not safe on typical pavement going that fast. On the hills I ride on mine was a watthour hog. I parked it for a geared hub motor.
If you have a steel front fork, you have the opportunity to put the motor on the front wheel as I did. Check the fork with a magnet. Cruiser bikes put too much weight on the back anyway, with not enough on the front. A motor up there helps balance that problem. Front hub motors should not be ridden powered on ice, wet steel covers or plates, wet wood bridge decks, or wet rock. Pedal on those surfaces.
Sometimes a motor won't fit in the slot on the bike. A little grinding out of the slot with a grit wheel was in order on my first bike. Use safety glasses with power tools.
Be sure to install a torque arm to keep the motor from spinning in the slot. I capture my torque arm with a clamp cut out of sheet metal, a box fan frame. I use 10-32 x 1 1/2 screws with elastic stop nuts that don't come unscrewed. 5 mm screws could also be used. Mcmaster or grainger or fastenal are sources of boxes of SS screws that won't rust up. People that use worm hose clamps to hold the torque arm make videos on u-tube about coping with the twisted off wire harness.
You can install a PAS pickup on the crank if you wish, using a special crank arm puller. I had one, hated it, and deleted the PAS when I moved the motor to a 2nd bike. I use a throttle now, exclusively.
A rear hub motor is more traditional, but I had an 8 speed rear sprocket cluster, and couldn't find one with 11-32 range in 7 speeds that would fit on the DD hub motor.
Ebikeling sells basic hub motors built into a wheel already. Mine lasted 4500 miles before wearing out a gear. His displays are not worth the money, they show speed zero when the power is shut off downhill. A more high performance Mac10t (fast) or mac12t (torquey) can be bought from EM3ev in HK. My mac12t will run me up to 23 mph on the flat with a 48 v battery. I would use a throwaway bank card if I was sending money out of the country, instead of my debit card linked to my bank account.
Be aware, the power wheels tend to require a long stem tube, available only on the internet.
Generic power wheels can be bought on ebay & amazon. I do not recommend batteries from either source. I got money back for the amazon one that failed in 1 day, but the ebay one it took me longer than 31 days to prove it was garbage. Em3ev is reliable on batteries. Or see littakal that has a thread running on parts & accessories. If they make it through shipping, they seem to work. Getting them through DHL seems to be a bit of a challenge, however.
36 v seems to be where the market has moved for geared hub motors. There is a problem in the states with the sierra and rocky mountains, with purchasers of geared hub motors burning them out on the 2nd trip to the park at the top of the mountain. That is probably why ebikeling stopped selling the 48 v ones. Don't climb tall mountains with a geared hub motor. but I climb 80 hills on my 30 mile commute with no problem. 200' overall rise in 3.5 hours. 3 or 4 hills are 15% grades, but <100' long. Interspersed with downgrades.
As far as wiring, I used 3M or dorman crimp terminals with a klein tool on the phase & battery wires. Just match the colors. I used a lot of tie wraps. If you leave 18" of slack on the hub motor end, you can cut some ties and change a tube the normal way, without unwiring the motor. I carry a dozen ties, which weigh nothing, in my repair kit. If you don't have a display, you don't have to worry about turning the bike upside down to change a tube out on the road. I found the statistics of the display stupid. I know how far I went from the map, and I know how fast I went with a wristwatch. The only useful item is battery voltage, and that is built into most throttles these days. At least a green/yellow/red battery state LED.
Happy modifying.