RODI unit: is this one any good?

Nope. Get the CSP-DI, then just plumb out of what's there. You'll see a line going from the RO to the DI (it's exposed), you just need to T it yourself.

 

That's better than I thought.

 

Its about $20 for a prefilter and carbon block too. You are talking apples and oranges though, Pur and Brita remove chlorine so water tastes good, RO and RO/DI remove TDS and everything in the case of the RO/DI. There are no similarities and they are not interchangable, there is no substitute for RO/DI period. If all I want to do is make good tatsing water I would forget the Pur or Brita and use a single carbon block, thats all you need.
Get a low range chlorine test kit and I'll bet $5 you are exhausting the Pur filters a lot quicker than you think you are. Its similar to the people who buy a API Tap Water Filter and swear they are getting hundreds of gallons out of them when in fact it was exhausted at between 15 and 30 gallons and they are adding tap water to their systems. Without the test kits or meters to monitor the equipment you might be doing more harm than good.