you want to try and get a quite high kw output combi preferably say a 30kw like one of the worcester's, the higher the output the better ltr's per minute you get of hot water. if you spend more on a good make now like everything you will save in the future mate trust me.
if you get a good output you wont have a problem, the 30kw will run two taps easy
where about are you based mate?
luke