Same old: give a couple a go! Most have excellent reviews (a quick google comes in handy). Just be proud of the bike you choose, whether from ‘everyone elses recommendation’ or trying one out at a centre and buying on the spot!
For 100 mile epics – unless your planning crazy stuff – go with a single pivot or DW design. Less moving parts/bearings! & (generally) stiffer to pedal up.
Dude I bought a Zesty 314 in 2010, I’ve done everything from pretty hectic downhills to 24 hour pairs at bontrager 24-12 to 3 day epic bike packing rides – its done them all without fuss and just keeps coming back for more.
The longest I’ve ridden on her in one day is 90 miles but it was my legs which gave way long before I even started to get uncomfortable on the bike.
Couldn’t recommend it highly enough – plus with £3k burning a hole in your pocket you could get the 514, not everyone’s colour choice but some really nice kit on it.