Welcome to the forum!
1) All the 4-strokes are heavy duty compared to the typical 2-stroke that's used. The chinese 4-strokes are copies of Japanese industrial engines, usually Honda, so they are quite good for a "made in China" motor. I have one and it's a little underpowered compared to the Honda GXH50 it's copied from, but it's reliability amazes me. It has needed nothing but fuel and an occasional oil change for the most part in 13 months.
2) Yes, but there are 2 different 4G boxes. The "Honda" 4G is narrower and pretty easy to run on the SBP 4-stroke shift kit. The regular 4G has a domed clutch bell and is roughly 1.5" wider. You might run into crank arm/pedal clearance issues using the wider 4G. Custom arms can be made to solve this.
3) Starting with a good bike is ideal of course. Cheap new bicycles have generic components which will likely cause you problems down the line. Many members here will agree that it's better to buy a quality used bike than a generic new bike.
4) You cannot use a coaster brake with a shift kit anyway. No loss there!
Best way to accomplish a shifter 4G:
Start with a bicycle with good brakes, 5-8 speed external shift or 3-14 speed internal shift. Must have enough room for a 9.5*9.5*9.5 box in frame.
Buy Honda GXH50 or Huasheng 142F with "5/8 shaft"
Buy Honda 4G standalone
Buy SickBikeParts 4-stroke shift kit
Then you need throttle and fuel tank.