We're in a similar position - in the market for a new boiler.
Had a plumber/gas fitter in today and his take on it was that to fit a condensing boiler we'd need to replace all old rads and pipes (ranging from 8 to 25 year's old). The reason being condensing boilers have finer tolerances within the inetrnal workings and would end up clogged from the sludge of an old system - regardless of how much the old system was flushed.
His price for fitting a new, standard gas boiler was €2800, which I thought was outrageous!!
I don't know about the rad size/temp/efficiency scenario, but replacing rads alone - if the pipes are 1/2" to the rads - shouldn't be a problem. New rads are cheap enough - pick up a catalogue from B&Q or Heat Merchants - it's the fitting that blows out the price. Fitting a new rad however should be easily within reach of an average handyman.