Hi James & welcome to the forum
There is a similar thread -->
http://c5alive.co.uk/forum/index.php?topic=39.0that pretty much answers your questions.
In short, if you stick with the original ignition switch you limit yourself to replica Sinclair batteries at about £85ish from Adam Harper. Or you can use the closest matching commercially available battery - a type 038. The standard grey Sinclair battery top just about fits,leaving you free to use the key switch.
Most of us here use a wiring kit , available through the c5alive shop, that opens up your choice to leisure and car batteries. My personal favourite at the moment being a Halfords 70ah leisure battery.
Hope this helps.