I live in a Soft water area, I carry tap water in my tank and process it as I need it. The set up is Tank, pump Di, Di, pole. I have a double DI system to save resin costs. My TDS is 70 and Im on a water meter so its cheaper to use a DI set up.
If you get a fascia clean or a pressure washer job you dont need pure water so just by pass the DI and use tap water from your tank. 
Absolute spot on, same set up here. I also would advise 2 x DI vessels, this will save you loads in resin. My tap tds is around 80ppm & i use 2- 25ltr bags of resin per year- thats 160 quid all in! (400ltr van mount)
With a tap tds of 45ppm i wouldn't even give an RO a second thought, I would also use dowex or tulsion resin as these have far more life than purolite.
Also, being in a soft water area, if you run low on water you can top up from custy's tap if you set up your system this way.

A no brainer IMO.
tony