water Rain barrels with attached drip irrigation

This year I put in two 55 gallon rain barrels to harvest rain water. I am planning on connecting a drip irrigation setup of some sort. Does anyone have any experience with this type of setup? Here's my shopping list so far:
 
Filter - to catch anything in rain barrels
Vacuum breaker - to avoid back flow into barrels
Pump - I'm unsure how the water pressure will be so I am assuming I'll need a pump of some sort for even water pressure.
Fertilizer injector - I'm still researching this. I'm leaning towards the EZ-Flo 3/4 gal but I'd like to hear some feedback on injectors
1/2 inch poly - main runs
1/4 inch poly - runs from main to plant base
1 gph compensating emitters - not sure if this is the correct size or not... looking for feedback
 
I know I'm missing stuff to make this all work. Please chime in if you have ideas, feedback, or advice. Thanks!
 

I ran my rain barrel setup on gravity. (no pump)  I made my own drip rings using a 1/16" drill bit, no more than every 6".  Lift the barrels approximately 3' above your watering level, and you'll never have to worry about being able to get the last drop out of them.  even easier to get more height if you mount them horizontally.
 
Just a word of advice - if you don't need to complicate it with pressure, don't.  Make sure that you have a very good reason.  Once you put that pump in place, you've just raised the cost and complexity of everything down the line.
 
I run my gravity system with nothing more than a sand filter.  It has (2) 2-1/2"  Uni-Seal grommets that connect the barrels, with an overflow tube inbetween, so that I can divert overflow water into my swimming pool when it needs filled.
 
Don't mean to be a Debbie Downer, but your rain barrel irrigation system is going to be a very expensive startup cost, and will require almost constant maintenance.  I have to constantly re-fit my drops, because the warming/cooling makes the poly lines lose fit on the barbs, so I have to go through the whole system, trim the lines, and refit.  Works OK with gravity, but with pressure, you'll be chasing blow-offs.
 
That being said, your compensating emitters all depend on what type of end fitting you use.  If it's a simple drip, 1/2 GPH is pretty standard.  If you want to run any type of spinning sprinkler, you want to 10GPH.  2GPH is nice for drip rings.

In our drought I've collected rainwater for the blueberries, at least 4,500 gallons. I doubt that will be enough till our first rains next +/- November but if lucky we might get some sort mini storm before then. From the roof, I can collect approx. 200 gallons from each tenth of an inch of rain (after the roof gets wet). I use no complex filters - the water is only going into the garden.
 
My delivery system is extremely simple, but does require a little hands-on work to water. From whatever container I'm drawing water, I drop a submersible pond pump (GFI outlet) on which is attached a regular garden hose. I mix whatever fertilizers in the containers, usually 2 good quality 32g trash cans, and water the blueberries in pots. I have to move the pump once unless I'm using a larger source container. That's it. It only takes 10  minutes or so. I use about 50 gals per watering, which is about every other day.
 
Since water tends to have a mind of its own and can be very unpredictable, I'd suggest you start with a very simple system and then go  more elaborate from there.