Application on Different SQL Servers
Unlimited everything?

Sure, buy five different physical boxes at $50,000 each hardware alone, use Core-based Enterprise licensing with SA, patch them all, administer them all, and pay annually for the licensing.

Realistically?  In a way you can justify to accounting?

Use a physical box either directly running Windows and SQL Server, or via a hypervisor, 


give it plenty of RAM (enough to fit your entire DBs if you can afford it) - if you're stuck with Standard Edition, 128GB is the max (leave some for the OS)
a couple good, fast CPU's
and put your data on fast, reliable enterprise solid state drives - either local or on the SAN.
I do agree; do not put your actual applications on your SQL Server system/guest.


If they step on each other, look at the Resource Governor or separate VMs.  Watch out for licensing costs on the latter!