Hello and welcome to Beoworld!
What are the speakers that came woth the Beocenter?
As far as I know your existing speaker cables should work fine with the redlines.
Almost any of the range (except possibly the RL35) would meet your needs. Some models suffer from foam rot in the woofers, but age/condition and price aree as imoortant as the specific model. Bear in mind some versions of the redlines are huge, so look up the measurements before you buuy, or give your wife plenty of warning!
You should also consider the S45-2 as this will probably be a step up from what you have now and they can usually be found quite cheaply.
Simon