I am in the process of researching this very topic for a 2016 Ford Escape. Its a FWD vehicle that I want to "simply" power the rear wheels via an electric motor more for performance than anything, but it should also help my MPG some (which actually isn't bad for the power the 2.0L Ecoboost puts out. 😜)
I have looked at different options from hub motors (not viable at this time nor are they available) to connecting a Hyper9 directly to the rear differential to my preferred option of retrofitting a full Tesla Model 3 rear suspension with small drive unit in place of the existing unpowered rear suspension. Connecting an electric motor (AC or DC) directly to the rear differential will likely not be a good solution due to the gear ratio and generally poor performance. Basically the motor won't spin fast enough to make sufficient power unless you had a shorter (numerically higher) gear ratio, something in the 6:1 or higher. Tesla uses around a 9:1 differential ratio in their motors, so that gives you a frame of reference if you are not using a manual transmission. A 2-speed Powerglide transmission or the EV Torquebox combined with the rear differential should allow for better power and performance without a traditional transmission, but that is added cost and complexity.
Some of the key issues I have been sorting out have to do with integrating the 2 separate systems and controlling them without having two separate accelerator pedals. I have a drive by wire/electronic throttle body setup so my gas pedal has basically the same 6-pin connector as the OEM Tesla unit. That way I can split that signal and send it to both the combustion engine as well as to the motor controller for the Tesla drive unit. The same challenge exists for the brakes if you plan to have regen braking. Depending on your setup, you may get by with a small pressure transducer that plumbs into the stock brake lines, or in my case the brake pedal position switch that my car already has should provide the signals that I need.
The specifics of the motor you choose will drive the controller, but none of the reasonably available controllers are designed to be used in a hybrid setting - they are meant for pure EV use. MoTeC makes a system that they market as a full hybrid control, but it is very complex and likely very expensive. You will replace the existing ECU with a MoTeC version that will double as the electric motor controller - but you have to provide a tune for the ICE, it doesn't come with one.I am planning to use an EV Controls T2C controller that has quite a bit of user adjustment over the motor output so I can "tune" it to work better with the ICE components.
Battery pack size and placement is also a challenge with a hybrid. Since you are not replacing some of the big ICE parts like the engine, fuel tank, etc. you will have to get more creative in where you place the batteries, BMS, and charger. You can skip a DC-DC converter if you plan to always use the gas engine, which will simplify things like air conditioning, power steering, and brakes. However, some chargers also have a DC-DC converter built into them so that helps to future proof the build if you are thinking about full electric in the future. I am planning to use a modest sized battery pack in the 20-25kWh range to get the necessary voltage to run the Tesla drive unit, and will package it under the cargo area floor and in the drive shaft tunnel under the vehicle with a protective plate to prevent any road debris damage. You will need to account for cooling lines and pump to keep the batteries and motor/inverter temps in check.
To build a hybrid, you are 2/3s of the way to a full-electric conversion, so it seems many people bail on the idea once they realize what all is involved for the limited MPG improvement you will likely see. (This site is litered with posts on the topic that whither after just a few comments.) Not to mention the challenge of getting the EV and ICE components to play well together. I spoke to a lot of different aftermarket EV controller manufacturers before I landed on the EV Controls T2C and its still not perfect for my application, but it should be a reasonable accommodation to achieve my desired outcome of a "sleeper" dual motor AWD 500+ HP Ford Escape. I encourage you to go for it, but you need to be honest with the challenges you will face.