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If it is just a straight swap your easiest will be a Solo (or Promax) as they don't need perm lives or over runs.
I normally use Worcester Ri's or CDi's depending on the space and the budget but have also used Ariston Clas and Alpha CDR's. Never had any problems with any of them. They all cost much the same (CDi excepted).
Don't touch an Icos. You can get a Worcs Ri cheaper anyway.
We recommend City Plumbing Supplies, BES, and Plumbing Superstore for all plumbing supplies.