All boilers sold in 90's will be the conventional type and all sold today are of the condensing type. Condensing boilers have more electronics and other components that may affect reliability. At a minimum, the system condensing boiler will have an electric fan and a heat-exchanger as an example.
A combi boiler does away with the need for a hot water tank and also a separate pump. When converting to a combi boiler, the work required should be minimal, although the costs can mount up. One issue with a combi boiler is that it may not be able to keep up with the short term demands but has the advantage of reducing space and simplifying install.
The most common issue with a boiler is that it is seen as not being efficient. A Scottish friend, assures me that there is no real reason to be pressurized into replacing it on cost grounds since you have to pay back the additional costs. When it is seen as being uneconomic to fix, then the condensing type is really the only way to go. There are still different types of condensing boiler depending upon how much you want to put in the one unit.
Most new boilers come with a 10-year service plan which means that you pay for the boiler and then pay for it again, in return for having it annually checked/serviced. My last boiler was an old-fashioned cast-iron unit. Not very efficient but lasted 30 almost years. Most new boilers will be lucky to last 15 years. I did once come home to a flooded utility room and had to spend £6 replacing four O rings that connect the two cast-iron halves, that had perished. My new boiler didn't like the condensing pipe filling with rainwater (an installation issue and heavy rain). When it filled/flooded the repairs (covered by the 10-year warranty) have required 3 visits and most of the "internals" to be replaced. Old-style boilers will (mostly) just keep going.
