I have travelled Iceland & Norway with a standard alternator.
I run a 110 200tdi twin optima's (Red & Yellow) on a split charge & a warn XP9.5 Winch. Also have 4x roof spots.
I have never had an issue with charging from the 35amp alternator.
With the expedition you will probably be travelling for long periods which will enable a good charge from the alternator.
If you do upgrade try to keep everything as standard as possible...eg alternator connection...& keep the alternator you have now as a spare
8)
I am sure Trayback Dan is running his competition vehicle on a standard alternator. Twin 8274 winches & twin yellow top optima's in parallel IIRC...send him a PM. 8)