I do think the ones I mentioned would make a great basic, beginning phero stash.
IG, IH, IS, A314, and IO, (and definitely Ammo, if you can ever catch it in stock 
) but you may not be able to afford these all at once.
Besides, I think you need to make sure they actually work for you, just start with 2 to-go sized products (which is just about what the $25 gift certificate covers), before you start to seriously investso maybe start with IG and IH (and maybe IS for a back-up).
Yes, they do work in combination, but it is extremely important that you become familiar with each product seperately on its own, and understand the effects, before mixing with others.
Steve wrote this very helpful post on the subject:
There are many great combinations, and its tons of fun, once you understand how they work andf how much works for you, everyone’s different.