Dylan, besides the bike itself, so the only other thing that cost me was the engine kit - which came out to around $150 I think. The kit had everything I needed in it to start, but it's like any hobby - you can eventually end up spending way more on it than you'd like to admit
Anyway, you'll no problem at all with that stock coaster brake - coaster brakes all have freewheels inside them that will let your rear wheel spin without engaging your right-side chain. So for example your feet can stay flat on the pedals while you roll down a hill. Same thing happens when the engine is driving the bike, you don't have to spin the pedals.
And as far as stopping, your coaster brakes will function like as normal after you add an engine - you just pedal in reverse, and the brake engages.
Good luck and have fun!!