Installing a high efficiency "combi" boiler in an old house
Fit the new boiler where you want.
As part of the job get the plumber to flush the pipes to remove silt.
The boiler does not have to be central - the pump provides the push for the water and heating circuits have balance valves to control the flow or they can be fitted.
But consider if the existing boiler still works, how long is the cost of fitting going to need before it is paid back by the increased efficiency?