Hi Bill,
There is a lot of choice in the upgrades.
For a standard Universal Carrier Mk II, then this looks like a good set giving you the option of a Command or Standard Carrier: I haven't built this set yet but it looks to be as good as the rest.
http://www.resicast.com/pages/35197_page_1pag.html Other conversions are for the Vickers MG Carrier or 3in Mortar carrier and the MK I Universal Carrier, which is a nice unusual one. An interesting alternative is the AOP Carrier which comes with an open engine and is one I've yet to build although I reviewed the kit here on site
The Bren carrier above would be suitable for the BEF and early Campaigns in N Africa etc, as could the Mk I which soldier on into Normandy as well.
Hope that helps, BTW they are addictive
Al