Some best practices are to run them separately, however, in doing so you lose out on the benefits of having a converged network. This can be important when you have a large environment and oodles of 10Gb ports.
However, your environment is a very small one, and I think Dell is trying to oversell you on network hardware, and their own iscsi hardware.
You can purchase a switch with multiple heads that is functionally equivalent to having 2 switches. Also, you can easily look at FC instead of iscsi, and maybe compare NFS too, along with infiniband. You can also use something like infiniband virtualization e.g. Xsigo?
On the NAS/SAN side, i would not be so tied to Dell, but might instead go with a best of breed product line, including things like Netapp and other competitors.
Questions I would ask:
How easy is it for me to find talent for this configuration?
How close to industry standard is this hardware?
What are my out year costs of this solution going to be? (TCO)
How expandable is this solution?
Does this solution miss any nice to haves?
Is the vendor trying to oversell me on a specific solution?
Do I adequately understand the problem space, and do I know (reasonably) a good number of alternative solutions?
Can I use one vendor quote to get price concessions from vendor 2?
How remotely manageable and monitorable is this solution?
How well does the entire stack integrate?
What is the cost per minute of an outage and how does that compare to extra hardware?
Can I mitigate risks another way?
Might I have better advantages by going with a cloud stack from a vendor in a regulated environment and trading off higher operating costs for less capital investment?
Where is my application aware security?
How easy is it to secure this infrastructure?
Am I attempting to optimize the solution prematurely?
Have I performed sufficient performance analysis and benchmarking to know what my true performance requirements are?
How does this system fail over and to whom? (HA and Vmotion, among others)?
Do I have single points of failure?
Have I received quotes for both integrated stacks and best of breed stacks, from at least 3 vendors apiece (6 vendors total)?
Can I go with a different model altogether, perhaps using a blade enclosure with blades, or virtualized i/o over a higher speed network (Xsigo)?
Can I use virtualized switches (e.g. Cisco 1000Vs and their competitors) instead of physical switches?
One other thing I would add is that several vendors are now selling pre-engineered solutions like the Cisco/Vmware/Netapp partnership with Flexpods, or what would be a competing fully one integrator solution such as HP's VirtualSystem. I'd ensure that these vendors know about what your goals are, and they get working with their own virtualization specialists to create a solution that meets your requirements.
You are able to use demo model solutions from these vendors without buying anything (for a limited time) then make a selection based on whichever best meets your needs. Head to head competition - always a win :)