2010 upgrade to 2013 - do we need separate servers for caching and office web apps

We are planning to upgrade our SharePoint 2010 farm to SharePoint 2013. Our 2010 farm consists of the following servers:

1. 2 WFEs

2. 1 Index Server

3. 1 Database server

We initially thought we would be able to upgrade it to 2013 without having to add new hardware but our SharePoint consultant tells us that we need two new servers. One for  Office Web Apps and and the other for Distributed Cache Service. General documentation on MS site does say that these services must be installed on separate servers. Can someone please guide us and let us know if it is absolutely necessary to have these services hosted on separate servers? Can we just enable these services on the WFEs? We do not want to purchase new hardware just to migrate our existing farm to 2013.

Thanks,

James