It can be confusing enough as normally I'd suggest you do a power audit to fully detail all you are likely to use to help assess your needs.
However, to keep it simple, a 100w panel should do the job fine. I have fairly low demand (TV, LED lighting, pump and phone/laptop charging) and my 80w panel does the job more than adequately.
Efficiency will vary during the year; obviously better output in the summer months than during the shorter winter days. Do you do much 'vanning later in the year?
There are plenty of kits around that include a charge controller (required to maintain battery during charge) that are usually stuck onto the back of the solar panel; not the best place for it for cooling and efficiency, so get yourself a panel with a separate controller to allow positioning this close to the battery.
The panel is arguably a high-risk item and many chain them to the van while in use. Freestanding panels are better from an efficiency point of view as the user can angle them for best use during the day.
If you use the search feature, there are loads of threads on the subject, but have a nose through this recent thread here too.
As G7ACU says almost as important is sourcing low demand equipment. Also depends on the capacity of your battery and how sunny it is to recharge it. adecent charge and load watt meter will give you an idea of charging and consumption
deffo chain it up to something unmovable,, i use a motorbike coil type rope to chain my 100w suitcase panel to my folding camper.. I never leave it out of sight either.. i have two batteries and as of yet, never have run out of juice.. If i think i have used more than im going to recoup from the panel, i have an inverter and charger in the car, so when we go out shopping i can charge up on the move.. No real reason to ever totally run out of juice really :)
i think my panel cost £120 on fleabay,, albeit i have thrown the controller and brought a better one..