I'm biting my tongue re those systems. I won't say it, I won't say it...
Here are some things to look for:
Standard-sized prefilters, membrane, and DI cartridge
Brand name, high-rejection membrane
Specifications provided for each stage
Pressure gauge after the prefilters and before the membrane
Thermometer
TDS meter (handheld or in-line)
Vertical DI stage
Refillable DI cartridge
Rust-proof bracket
Quick connect fittings
No water from pressurized storage container delivered to DI
Flush valve
Clear housings
High-quality instructions written in clear English
Customer support before, during, and after your purchase
A vendor involved in your hobby
Russ