Hi Zen
Forget the 50 gal....go to a ag supply store or large hardware and order a 1000 gal stock tank.
When you fill 55 gal drums there is alot of transferring of water and they really will overflow immediately. Put screens or hardware cloth into the downspout to prefilter. There is usually a hose connection at the bottom of the tank, but a pumped system is better. Screen all openings and make sure to provide a overflow hole (screened) at the top.
1000 gal tank will keep water for the dry spells, 55 gals will be used up in one watering.
It is hard to explain, but there is a great leaf filter design. In the downspout place a angled screen with a opening for debris to fall out and the water to pass straight thru. it can be made with a pvc cleanout fitting. (wish I had a picture).
We have been living with rainwater collection for many years and it is our only water source for the whole house. (there is a 12,000 gal. concrete cistern under a part of the house.) We also collect driveway water into a separate cistern for watering and flushes.
If all openings are screened, it will keep out bugs and frogs, but make sure they stay intact.
If you do use 55 gal drums make sure they have been used in the food supply industry and not chemicals. Soft drink syrup comes in barrels, maybe you could find them
Nelson