Before we switched to my own pump, we used el-chepo pre-set plastic Raindrip regulators. While they are not lasting long (I've got typically two-three seasons out of one), they were cheap.
I had bad experience with plastic rain BIRD regulators - less than a season of use.
Many other companies make them, typically in $5-$10 range, like this one:
DIG Corp 25 PSI Hose Thread Pressure Regulator-D46 at The Home Depot
My preferred external pressure regulators are brass RV type ( this one: Valterra Lead-Free Brass Water Regulator
)- but they are typically 40-50psi and not 20-30psi. While 50 psi will work on Raindrip emitters, the emitters will not last as long, so go for 25psi.
BTW: If you need something really long lasting (think 30 years if protected from elements), use a "whole house" type regulator,
like this: Watts 3/4 in. Cast-Brass SWT x SWT Water Pressure Reducing Valve-25AUBS-DU at The Home Depot
As far as you question about regulators minimum flow - if there is not enough flow, most of them will still work good enough for the drip job, but can "buzz" (it drove DW bonkers) as they will open/close many times per second.
How many plants are you watering? For potted plants you could even use 2GPH drippers, as the potting soil is very quickly absorbing the water.
I also advise against using PC (pressure compensated) emitters as they clog much more easily (and you are going to use pressure regulator anyway).