Yeah you are nearly correct on this one, the "Edge Gateway" product is purchased as a single module, although you cannot stack this with other modules on an appliance, as it is already a combination of APM+WA+WOM (unless there is anyway of having 4 modules on a BIG-IP platform).
The bundle itself does not include LTM as it is targeted as a remote access solution and you cannot add LTM (for the reason mentioned above) so you are required to use a "Webtop".
If you require LTM they you will need to stack this alongside the APM module, this also creates another option (not included in Edge) that allows you to directly assign the "Access Policies" to the LTM VIP instead of using a Webtop.
"May I know what is the reason for the WOM module does in edge gateway since it is a remote access solution, and customer don't normally combine VPN and Wan Optimization in a single box.."
When we are talking of users remotely accessing the network, we are talking of the people coming over the WAN link where latency can't be avoided.Application performance on the WAN is affected by a large number of factors that include the natural behavior of application protocols and software that were not designed or optimized for WAN conditions, application protocols that engage in excessive handshaking; and the serialization of the applications themselves. WOM module is there to take care of such issues.... I hope the things are clear to you.