The larger system is almost certainly oversized, and the smaller one quoted is likely as well. You don't need to upsize the boiler based on the size of the indirect. Generally, it is set up as a priority zone, and all of the available heat will go to reheat the water when that is required. If you don't have a big soaking tub, you can likely get by with the 45g tank. But, if all three showers might be running, and those users like to take long, hot ones, you may prefer something larger. Sounds like that is unlikely, at least in the near term. You could compromise and go with say a 60g if you wanted to hedge your bets. The good thing is that, other than size, indirects tend to be quite well insulated, so you don't have a lot of standby losses like you could (relatively speaking) with a gas fired stand alone WH.
Without a good heat load analysis, sizing the boiler is all kind of useless.