I have just done this. You can see some pics in the gallery section.
So I have fitted a leisure battery to provide a 12V source for lights, fridge, radio and socket for a sat nav. The battery was £80 (75AH) from a caravan shop. And wire, fuse box and connectors probably around £30 from an online car wiring supplier.
You will need to charge the battery. I have an "intelligent" relay to charge from the dynamo automatically when it is spinning fast enough to generate enough oomph - I needed to adjust that myself using a multimeter. That cost £20 from bluebird.
I have also put in a mains charger. Now that's a minefield - I went an expensive route with an excellent CTEK unit, but prices are anything from £80 to £200 for a decent charger. You will then need to supply mains to that - could be an extension lead into the van - or you could plumb in a 240V mains system in the van, which is what I did.
You need to find some way of getting it in - I have modified the existing hookup socket, some people put a socket in the engine bay. You really must put in a consumer unit with an RCD - around £40 from screwfix, plus some decent wire (perhaps another £30 worth). I have used that to power a socket for the charger, and to give me a twin 13A socket in the van for mains powered appliances.
There are also specialist motorhome systems that control the various electrical bits and bobs - Google ZIG, for example. Then you start talking money.
You might find there's a local auto electrician or motorhome place that might be able to help? And yes, ask them what they are giong to do before they start drilling holes
Cheers,
Nick