Can't comment with authority on the failure rates, though I'm certainly happy with my 3.0L for power and economy. The discos, as you mention, are also king in terms of rear storage - square shape, and big.
That said, the RRS of similar years, with it's TDV8 is attractive, and if you can reorganise to utilise it's boot, it could well be a solid option. They also have Supercharged petrols..
The other option that may be worth consideration is a 'proper' range rover? If you're in the used market, and have some experience up your sleeve, you will know what you're getting in to, but could well be worth it.
That all said, sticking with a disco, in the 2.7L engine, means anything you've added to your old rig will likely transfer across (roof racks, dual battery systems etc)
2010 TDV6 3.0L Discovery 4 SE remapped to RRS output, Alaska White, GME XRS-330c, IIDTool BT, Dual Battery, Apple CarPlay, OEM Retrofitted: Cornering lights, Door card lights, Power + Heated Seats, Logic 7 audio
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