Everything on an RO/DI is just plastic housings and tubing, so don't break the bank. The membrane, cartridges and resin make or break an RO/DI system. I've got an old GE system that I upgraded with smaller micron pre and post filters, a spectrapure membrane and a replaceable DI cartridge with colour changing media. Works great, but I'm fortunate to have good water pressure here. If your household pressure is low, you need a system with a booster pump. I use a portable TDS meter, but again, it would be nice to have a built in 2 stage TDS monitor. The only thing I really would like on my system would be a clear housing on my prefilter so I can see when it starts to get funky (it's white opaque now).
All that said, I've never heard any complaints about the vertex systems, that's for sure. If I was in the market, that's likely what I would grab.