Depending on how much you intend to modify the car the warranty may not be worthwhile.
Obviously any spring or wheel changes are going to void warranty on the shocks. Any engine map, intercooler or downpipes are going to make any engine warranty worthless regarding the injectors or fuel pumps. To me the beauty of owning an N54 is the modding, they are very tame without a tune.
If you find one with injectors already replaced then I'd be putting the £50/month into a savings account for when something does break as that's the only guaranteed failure on anything older than a 2016 model. The cars are very easy to work on for a home mechanic.
I made good use of my warranty for its duration but I got sick of the "wait until it breaks down" approach, I'm much more pro-active with car maintenance. For example, I knew my injectors were bad for months before the car started throwing engine codes but the dealer wouldn't touch them until it started misfiring.