Hey Amrit and welcome to BimmerSport!
Bit of a old Subby fan myself but now a Bimmer guy
I think you should look at something like this: https://www.trademe.co.nz/motors/used-cars/bmw/auction-1439430561.htm
modernised LCI variant (arrived in 2007 - you can tell by the gearshift)
optional 19"s
Alu interior trim is the nicest looking
Features Dynamic Drive - this is a big help with handling on these big cars @Olaf has driven and compared E60s with and without Dynamic Drive and there's a noticeable difference
Hifi Pro - very good sound system
Bluetooth phone interface
HUD - this rocks - love it in my M5
looks like it has aftermarket exhaust tips
Includes balance of a mech warranty
In terms of an E60 sedan but not an M5 this is about as good as it gets. When looking at the exact spec, put the rego into Carjam to get the VIN, and then put the last 7 characters into www.bimmer.work/ to get full detail.
The 550i has 270KW which is a good stack of power so I don't think you'll be disappointed.
This one also looks great: https://www.trademe.co.nz/motors/used-cars/bmw/auction-1395272527.htm - very similar but in Akld and $2K less ..... but with std 18"s and wood trim. It's with a dealer so definitely take it for a test drive.
Costs/risk ...
Don't expect lower service costs, and in my experience (admittedly with older/higher kms BMWs than these) with age the plastics and rubbers deteriorate and can require costly repairs - not so much parts as labour.
My advice is always get a warranty/mech breakdown insurance (Autosure/DPL is brilliant)- and keep some $$ in reserve. NEVER spend all of your budget on the vehicle.
The biggest things to be aware of is valve guide seals - and make sure your sunroof drains aren't clogged or you can soak your car interior (yes really).
Cars with detailed service history are strongly preferable
always get an expert pre-purchase inspection done
There is a very useful guide to E60s attached.
All the best with your shopping process!!! I think they're fantastic cars.
So You Just Bought an E60 (V3.0).pdf