Coaster brake are a pain in the butt to install but...
I like them because I can apply it with with my foot instead of having more levers up front. If you buy a cheap cruiser, I would take off the front fender and put on a second hand brake immediately. It is an inexpensive addition but nice to have for better control and for the inevitable bike chain break at the worst possible moment. A coaster brake and a front hand brake is ideal in my opinion, which is worth less than some of the others here.
As for kits, I have the boy go fast kit from ebay. If you assemble it correctly it seems to be fine. I assembled mine wrong and it was a mess. Go slow in your assembly and consult this forum for instructions. If you have questions ask here. The people on this forum will actually answer technical questions and be happy to do so. You might even get a laugh or two along the way.
As for the additional work for the coaster brake, it amounts to enlarging the hole in the new rear sprocket. I drew a circle around the hole about a quarter inch thick. Then I went to home depot and bought a metal rasp for my drill. I sat down and enlarged the hole without any fancy tools or expertise. It isn't perfectly round but it doesn't have to be. It just has to be more or less centered so that the sprocket can be centered on the rear wheel. YOu also have to bend the coaster brake arm enough to clear the bolts again not a big deal if you have access to a vise. I didn't so I bent mine with a sledge hammer and a big pipe for an anvil.
One thing to remember when you get frustrated is that even I could do it, so you can do it to.
Do not believe that you can install this kit in four hours, that is someone's marketing tool, but it is well worth the time and effort to see the look an people's faces when you cruise by them. Not to mention just shear fun of a slower paced world you actually feel part of again. You aren't insulated in a rolling bubble on these bikes you are part of the enviornment as you tool along.
One more thing on the bike, oversized tubes look stronger but they are harder to fit the engine to. A nice standard sized tube is best. Get a bike with the standard 'slice of pie opening' in front of the seat down tube, if possible for ease of fitting. 26" is better all the way around. Though my first install was into 24". the bike looked great because the complete opening was full of engine. It looked more like a mini motorcycle but alas it I looked like a clown on it lol. Not only that it wasn't as stable as the larger bike.
Welcome to our world..
Oh if you are handy with tools look at the bike thrift shop and you can build a custom bike for peanuts. In my opinion its a lot more fun but also a lot more work.
My transylvainia chopper built form several junk shop bikes. parts cost probably under fifty bucks with lots of parts left on the curb for other builders to pick up.
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Note my reference to 'first build'! It is very easy to catch this bug... for me, much to the chagrin of my wife! 
