When compared to older boilers, the most modern types of boilers are significantly more efficient, while they run - but the modern ones (again by comparison) have a rather dire history of breakdowns (including "not worth repairing - replace"). Look at the price you are quoted for the boiler and how long the guarantee is and assume it will need replaced at any time after the end of the guarantee (add that to your cost calculations). You can buy insurance against boiler breakdown and then add that to your fuel bill.
You are unlikely to get a definitive answer as to "top of the range" and even then you will not know how soon and at what cost you will need to replace it. I suggest you concentrate on how long any one candidate boiler model has been in production, its available (reliable) durability record and additionally get comments from different installers.
The true cost of more efficient boilers through breakdowns and terminal failure is something that seems to go largely without comment. Instead the focus is on notional fuel efficiency and modernity with each manufacturer bringing out a flashy new model at short intervals and the history of the previous one gets buried. By all means insist on buying the newest one claiming to be the tops, just make sure you are happy to pay for its total lifelong cost. I know of boilers which, yes, are less efficient but in the 30 years or so of use the total repair cost for each one has been at most £300 over the whole of that time. I know of modern boilers which had to be replaced after around three years at a cost of somewhere between £1500 and £2000 - that translates into adding £500+ annually onto the fuel bill.