Unfortunately, it's not really feasible to convert that particular bicycle to use a coaster brake. That bicycle has vertical dropouts, which means that the chain is kept taut using a derailleur or other spring-based chain tensioner. A chain tensioner only applies force opposing the forward direction, however, so when you attempt to pedal backwards to actuate the coaster brake you'll only end up bending the tensioner back around and dropping the chain rather than stopping. (Without a tensioner of some sort it's extremely difficult to set the drivetrain up with the correct amount of slack; sometimes involving chain half-links and filing the dropout can get you close but there are no guarantees.)
Depending on your preferences, you can get up to 9 speeds with a coaster brake using the SRAM i9 internal hub; cheaper options also exist for 8-,7-,5-,and 3-speeds. I personally have a soft spot for the Sturmey Archer 3-speed coaster brake for it's balance of simplicity and a relatively wide (33%) gear spacing, but your mileage may vary.
Your best bet is to look for a relatively inexpensive single speed, fixed gear, or already-coaster'd bike to start from. If it's got horizontal dropouts or track ends you're in business. Frames purpose-built for coaster brakes will often have a tab brazed onto the non-driveside chainstay for screwing the anti-rotation arm to (lest the brake simply spin the hub backwards instead of stop you) but a p-clamp or hose clamp can be used as an adapter trivially.