I'm going to fit a leisure battery in Vanny (LWB hightop Transit connect) shortly, I know which battery to get, just not sure what other stuff to buy -
Loom to connect into existing battery/alternator?
LED lighting and wiring?
USB charging socket?
12v socket for a ceramic heater?
Any recommendations/links to conversions are greatly appreciated!.
And all the best for 2016, happy biking 
Definitely need a split charging system and the rest depends on how much you want available in the back.
USB charging for gadgets and lights for when you're using the van in the dark are always going to be useful. Probably worth fitting a couple of normal 12v cigarette lighter adapters too as most mobile stuff will plug in to those.
You might want to consider an inverter too for a normal plug. My friends just had a splitter installed with a plug in the engine bay for hook up so he didn't need to cut a hole in the van.
Look up split charge relay kits on eBay
There's also a neat mounted 12v cigarette socket and USB dual port on eBay. Will search and link
Don't bother with a 12v heater, waste of electricity. Keep an eye out for a working s/h eberspacher diesel heater.
Inverter really only any use for laptop charging anything much bigger will just suck too much out of the battery.
Also led lamps and switch bank with Fuses a good idea.
Here
http://pages.ebay.com/link/?nav=item.view&id=351598613168&alt=web
Handy with voltmeter to show battery condition (14v when split charging, 12.7v is healthy, <12v without load not healthy)
I have 3 12v sockets, and 2 usb sockets. That allows DVD player charging, phone charging etc...
Never felt the need for inverter and 3 pin plug, not sure what I would plug in that wouldn't draw loads of power!
I have 2 led lights, like a strip light with built in switch for easy fitting. And a little led reading lamp in the corner.
Don't buy a cheap split charge. I had a £20 that caused many issues. Fitted one for around £100 from eBay, no issues in 3 years.
It's worth taking a look at the split charge kits that Travel Volts do on their website.
They're good quality and not too expensive. I personally would avoid the cheap eBay specials as I'd prefer something that won't pack in after a couple of months or potentially go up in flames!
I fitted a Travel Volts kit in my T5 and I've been impressed with it so far.
Also, remember that a split charge system won't be enough to keep the leisure battery full topped off. You'll want some sort of mains or solar charger too.
I disagree that split charge won't keep it topped up.
I don't run a fridge which is probably biggest drain of power for most vans. Only have lighting, eber heater and DVD player. Never had the battery die, only ever use split charge.
I disagree! Leisure battery can easily be discharged when camped up without some additional charge.
I'd add hook up point, garage consumer unit and double plug, plus a zig unit to the leisure battery and split charge relay.
Zig unit or similar runs the charging of the battery between alternator and hook up and then acts as fuse board for your 12v circuits.
In my Sprinter it has a neat steel box into which the leisure battery and zig unit live. You could form the same from wood and board under the Tranny seat, just remember to vent it.
It very much depends on what you're drawing off the battery.
A 2A USB port for phones/tablets, a couple of A of LED strips, a 12v radio and you can make a 110Ah battery last a week,
Stick a fridge on it and you'll be done in 12hrs.
I have a fridge in the campervan and have to monitor charging and use closely. It also has a Zig hookup & charger for use at campsites. But if I were going wild, I wouldnt use a fridge, just lighting and media and really wouldnt worry much about the state of the battery.