2-3hrs easy. Only 3 pipes and few cables on a heat boiler. Even if it’s a biasi combi and the pipes are in different positions, it’s not a big job to swap them round a bit. If i’m doing a swap, i’m normally put the x400 in at the earliest opportunity, usually the week before and let it circulate, i’d give the header tank a good scrub out first too. Then flush it out from each downstairs rad.
Best depends on your budget. Cheapest, not many options anymore, heatline have the best service.
Mid price, currently I prefer the Ideal Logic plus, 5 year guarantee. Vaillant have improved a lot in the last couple of years, not keen on worcesters, everybody I know with one has had problems.
Better quality, Viessman or maybe Alpha.