Multisite or Multi Template

Hi, I was going to put this question in the Multisite forum but decided to put it here as we are not yet running a Multi site as we are not sure this is the way forward.

We have our WP site set up http://www.myeloma.org.uk with Buddypress, BBpress and about 3000 users / 10000+ forum posts / 100+ pages and we want to extend the site with an e-learning section that has its own template and its own domain.

First we thought Multisite was the way forward but we are hampered by some issues with wildcard domains (in Plesk) and also not sure it is really necessary as all we are sharing between the sites are users. All we need is for WordPress to load a different template where we can set up a different menu system and the necessary pages and then somehow start WordPress with this template when visiting via the e-learning domain.

We have found tutorials for older WordPress versions (ie. MultiDomain) that may do this but looks like it may not be compatible with latest WP.

Can anyone advise us on whether this is possible with or without plugins or if there is a better way to achieve what we are looking for?