Worcester or Vaillant seem to be very common, they are up there with the best.
Powermax boilers tend to be quite good on hot water flow rate, so you may want to chose a combi with a hot water store inside to keep a similar performance. I'd go for one with at least 30kW capacity, even 40kW say the Worcester Greenstar 40HE.
Assuming that the powermax is upstairs and the new boiler would go in the same place, so a vertical terminal would be needed, a very rough guide would be £2000-£2300 + VAT
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