Anywho, one question that comes to mind is this: If your current panel has no more room, will it also have to be upgraded to one with bigger main breaker & bus bars to feed the sub panel? How about the utility line feeding the house? If the main needs to be upgraded for higher current service then the 2nd panel probably becomes moot because a new panel will have more slots for circuits...
This and to follow up.
is your main panel 200 amps with 40 circuits
or 150 amp with 32 circuits
or 100 amp with 24 circuits
as for no pdu let me link to you what works
should do 2-3 s-9's
http://www.ebay.com/itm/Dell-PDU-APC-AP6015-Power-Distribution-Unit-0T834-with-Cord-Tested-Good-/232099723175?
I'd have to check as far as the main breaker goes, but I ran the numbers and you can run four S9's on a 220v 30a circuit following the 80/20 power rule. I'm also not too concerned with a pdu as it should cost around the same to simply install four outlets rather than just one and having to buy a pdu since the electrician will be here anyway.