You definitely need an inline filter, but you can get one for like $15 (with a reusable screen) any place you get irrigation supplies, probably the same place the barrel came from.

Unless you mount the barrel very high (say, stacked 3 high), you can't use it with a drip irrigation system or an automatic waterer. Not enough pressure. That's the main thing - you have to just let a hose drain out, or fill a watering can from it, so it takes a little extra work to water.

If you want to actually use rainwater as a water supply, think seriously about installing an underground cistern with a pump. I haven't taken this step yet, but you really need to isolate the water from sunlight, insects, and airborne debris if it's going to be stored for more than a few days.

I just saw a very attractive rack made from 4x4s. Just stacked alternately, rail-fence style. Held together by drilling a hole in the corner of each piece of wood and inserting a piece of rebar vertically in each corner. You could just as easily stack barrels in a system made that way, by making the cage larger than the diameter of the barrel and putting a "floor" for each barrel.