new build

You can easily add bolt-on brakes to these frames but they are not all that great. I prefer to weld on V-brake mounts cut off a parts bike. I use the bridge style front mount cut off a set of suspension forks because it is pre-aligned and easy to weld onto another frame. Sorry I can't post pics because someone (!) lost the camera last time she went to visit the folks....
V-brakes are much better than regular sidepull and well worth the effort to install.
I'd still keep the coaster just because it's there and more is better. Re-grease the hub(s) well with good hi-temp grease for better durability.

I almost bought me one of them from walmart,,then I found that yellow Trek..lol.
That build should be pretty straight forward other than the front mount may not reach the tube,,so you will have to use a plate with holes drilled and a 1 1/2" U-bolt.Then the hub brake arm will need to be tweaked a little so it wont hit the sprocket,and the dust cover on the hub will need to be trimmed to fit into the sprocket,,but your better off making the hole in the sprocket a little bigger,,i used a drill press with a round stone and did a couple quick circles at the same speed to make the hole a little bigger.Oh yeah,,and as mentioned,,high temp grease is the way to go,,these bikes only come with a light coating of cheap grease.If you go with bolt on,,I used a set of the dia-comp BMX Bulldog side pull and they seem to work better than most other side pull,,they are a little pricey though,,think I paid around $20 for one set.