When it comes down to it these are all the same basic setup using standard cartridge sizes, housings, fittings, etc. Yes, there are "name brand" things like John Guest fittings and whatnot but I've personally never had a problem with other fixtures with no name as long as you don't connect/disconnect the lines all the time which the fittings aren't made to do anyway.
What you are looking for is the filter and membrane quality. You can get a cheap used setup for $50 and put $100 filters in it or $5 ones and change how it works. You can change the flow restrictor and put a bigger/smaller membrane or change the waste/product ratios. It's all the same housings.
I can't find the order now but I went with an
amazon special 5 stage RO for around $150 that came with spare filters and a drinking water reservoir and tap. No sense in just your fish getting good water. You'll have to replace the filters a bit more often of course.
After the RO unit but before the reservoir I plumbed in an Icecap Dual DI Filter for $40 for the ATO and water changes. Also added a 3 probe inline TDS meter.
You can easily get a very nice setup for both you and the tank for under $250 all in including an inline TDS meter addon. I have over 400 TDS source water with added chloramines. TDS after the RO is under 10. 0 after the DI. When the existing spare filters run out I will replace with 1 micron instead of the 5's that I have now but other than that I'm more than happy.
Another word of advice is to never fully trust the inline TDS meters. The probes can get twisted and skew the reading. They're also not temperature corrected. So ALWAYS do spot checks.