From 10 years experience irrigating with rainwater I would say that you can shoot for very simple, inexpensive, and easy to troubleshoot on the one hand or complicated, expensive, and a real pain to keep working properly on the other. I gave up automating the whole thing long ago, and now use simple submersible pumps with lengths of large diameter (1 1/4 inch) tubing to hand water. The flow rate is phenomenal this way so it is at least quick.

Most pumps don't work well at all with anything designed for the pressure you get from domestic water supply (i.e. that's pretty much everything including water wands, spray nozzles, and drip systems, even garden hose).

If you're looking for an automated watering system to take care of your plants while you're on vacation, it is difficult to stay on the simple, cheap, and easy side of things... One way might be to have a pump on a timer deliver water from the cistern once a day to a smaller raised rain barrel that is fitted with a gravity feed system of tubing connected to each plant. The timer would turn on pump just long enough for it to quickly fill the rain barrel, then the water would slowly be fed to the plants appropriately. The only difficulty with this would be controlling the flow so that it is the same for each plant, but you might find a way with standard drip parts. 20 pots is not too many to do something like this with.

I used a 55 gallon plastic barrel with a regular drip system from Dig last year for a vegetable garden and it did fine.It was on a little garden cart because it needed to be transported to the water source to get filled.I think it was only about 2-3 psi coming out of the tank but it was enough.The barrel was higher than any dripper in the garden.
There was no inline filter,maybe a screen at the tank hose valve and nothing clogged.It also didn't have a back flow preventor.Aren't those used to stop fertilizer or acid if used from contaminating the house drinking water.If so,maybe yours won't need one.
When it came time for fertilizing,I just calculated an amount of liquid or the water soluble kind and put it in the barrel after shaking it up with some water in a gallon jug.
I also had a cheap,battery operated timer at the tank. Brady

I did the whole automated rain water thing for a year before I had to rip it all out because of moving to temporary house. I did learn that the best spray heads were the ones that send out 8 streams water in a circular direction and each head can be adjusted for optimal flow. On 15 gallon pots and smaller 1 spray head was plenty but needed to be adjusted depending on how big the pot was. On my big 30 gallon pots I used 3 spray heads and dialed them back to achieve the desired wetting pattern. I got a 2.5 hp pump to run it and it did fine for 10 plants at a time. My timer was set up on 3 zones, 10 plants at a time. I had to run the discharge pressure at 40-50 psi to get enough flow to the plants. To achieve the desired pressure I had to install circulation line with a valve coming of the discharge back to the rain tank and adjust the valve position to get 40-50 psi going to the plants. After toying with it a while and getting all the spray heads adjusted properly it was very nice!!!!!!!! I will never go back to hand watering again once our new house is built and every things automated again. One suggestion is to have a manifold with a 200 micron screen/filter for every zone. I had to clean the screens once every couple months with a tooth brush. I hand watered 50 something plants every day for several years and I'm here to tell you that automation is the way to go!!! I will never look back. Stay away from drip. Rain water is too dirty and you will have nothing but problems with drip. 1/2 inch Polly tube and the little 1/8 hose to a spray head that adjusts flow on a stick is the only way to go. The spray heads need to be the ones that send 8 individual streams. I can't stress that enough as all the other fan heads and bubbler heads clog up all the time.

Chelesa,

I have about 200 BB bushes and I started out by dipping water from the lake I live on with my front end loader and hand watering lol.....those days are long gone.

What I did was add a 1/2 HP starite well pump, foot valve and float switch to prevent the pump going dry. The pump is mounted to the 300 gallon tank or you could have it next to the tank. I fill it 3 times once for each of my 3 BB beds. I mix acid in the water. It is basically the same as a well system for a home except for the float valve. You can also add a pressure tank to the system like a house on a well would have or a return valve back to the tank for the excess pressure.

Unless you have a very very BIG inline filter like a whole house filter I would not use drip as it would be a clogging nightmare for either the heads or a small filter.

You could just fertilize 600 gallons at a time and use one tank at a time with a valve to switch between tanks.

Have a friend with about 50 bushes in a double row all in half barrels so true 30 gallon pots. What we did was use PVC 1/2 conduit to make risers that the spray heads sit on, very simple. The 1/2 inch poly irrigation pipe runs down the middle of the row and the 1/4 inch line taps in to it runs up the riser to the spray head. each head covers 4 pots, simple and 100% reliable.

Nice, everyone! It's interesting to read about all the different set ups and gives me even more food for thought. I'll look into the circular spray heads and large diameter garden hoses :) I'm leaning towards automating because I know the day will come when I forget to water or just run out of time and my plants will totally dry out in just one day of not watering and die on me. I've invested too much time and money already for me to kill them by not watering daily! Lol