We checked out as far north up the Kapiti Coast as Te Horo (just south of Otaki). Otaki is quite a distance from Welly - which is why the property is cheaper. I believe (though do check this) that you can get the "Nice" train in from Otaki - it comes through P North - comfier than the Trans Metro.
At the moment - if you want to head in the Otaki Direction - you really do need to consider the train as your main option. The SH is full of roadworks, and there is a LOT of traffic using it - there are often holdups- and thats before you get past Porirua City or hit the Hutt Valley Traffic.
Also, bear in mind when looking at Kapiti, that Infrastructure cannot cope with Current demand from what I hear - and its getting more crowded, so services are going to get more pinched. Its all relative of course - many of these problems are actually not bad when you compare with te UK for example - and may be fine compared to Auckland.
Sizzlingbadger has nicely covered the Wairarapa! The hill is bad to start with, but it can be very spectacular. And you do get used to it.
My biggest peice of advise is to take your time to look at the two options. Thers actually quite a lot to consider between Kapiti and Wairarapa - each have thier ups and downs. Im really surprised we ended up here - it really was not what we envisaged - but when we looked at the options it was definately better for us. Do the drive, and take a train trip - see what its like and whether you can do that day in day out.
Something we are aiming for - OH spends 3 housr a day on the train. If he works while on the train for 3 days - thats an extra days work. He has effectively worked 5 days in 4, and can therefore work a 4 day week. Not everyone will have that option obviously - but it can drastically cut down transport time and costs (and give you more time to lounge in teh garden!)