You have one of the first condensing boilers ever built. The first generation condensing boilers HeatMaker and Glowcore- were not known for reliability, though there were obvious exceptions. Needless to say, the boiler is long overdue.
A proper heat load should be performed first, as your current boiler has twice the output of the radiation. Boilers should not be upsized for domestic hot water.
Modern condensing boilers are quite reliable if sized, installed and maintained by an experienced professional (someone you would trust in your house, even when you are not home). A ModCon with an indirect is one of the most efficient heat sources available and most comfortable as all condensing boilers now have built-in outdoor reset automatically adjusting boiler water temperature to the outdoor temperature improving operating efficiency and comfort at once.
Double-wall heat exchangers are going away for residential applications as they are totally unnecessary and diminish heat transfer efficiency. You may find an enlightened plumber who can help you with this. Proof of this statement is the fact that Buderus (the largest boiler manufacturer in the world) sells indirect water heaters in 150 countries - as I recall - and does not make a double-wall indirect water heater.
Which high efficiency condensing boiler is best depends on several factors including; local support, application, fuel available and radiation. For instance several European condensing boilers have operating water temperature limits, which may not satisfy a high temperature (radiation short) application.