What batteries do you get for your Series Land Rover?
Land Rovers original part is a 58 Ah battery for 4 and 6 cylindre petrol engines, and 95 Ah for diesel engined models.
58 Amp hour batteries might be fine for a bone stock landy, but if you have fitted (or have plans of fitting) auxilary lights, winch(es), and other non standard electrical equipment, then you probably want to upgrade to a larger unit.
You might need to swap out the whimsy dynamo for a beefier generator too. Especially if you plan on using a winch for more than 5 minutes at a time.