I have a relatively small, inexpensive RO syatem. (no DI) Almost all of them, mine for example, pass the water through a 5 or 10 "micron" filter, and then a 1 "micron" filter, before it gets to the RO membrane. Even after removing the sediment in the water down to that size, my filter goes through more than 50 gallons of water to produce about 13 gallons of RO. Then there's a carbon post filter used to remove any undesirable taste and any residual organics.
You could use just a 1 "micron" filter, but you"ll have to replace it more often. If you use just a 5 0r 10, that's a lot more water that will flow past the RO membrane, and down your drain.
Bottom line is, even the cheap ones work. You just have to do the maintenance. No matter how much you pay, you still, eventually have to replace the pre and post filters (I do every six months) and the RO membrane and post filter. (once a year). Won't mention where I got mine, but I "saved big money" at a local big box home improvement store.