This is the setup I use. Customers tap water strait into a 400 litre tank. Then pump the water through twin DI vessels then to the hose wheel. Oh yeah and the average tap tds around my way is 250ppm
I don't care it still works out much cheaper than renting a unit to make pure water, like you I live in a flat.
I buy 2 sacks of resin at a time, lasts me about 3 months. So for £140 I get a massive return on my money.