If you are short of resources (RAM?CPU) you may have to consider something that doesn't demand too much, such as the basic but very good (rules-based) Kerio 2.15.
But see how Online Armour goes. It has a HIPS component, too, and beyond that I don't know much about it, except that it's popular, well supported, and the forum is supposed to be helpful, with the developer taking an interest.
If OA is too demanding you may have to try something else.
