13hp is an awkward number of hp to be left with...
all the modules you mention would be good - it's mostly a case of fitting them in...
batumi and poti would fill your 13hp nicely... as would toolbox and rnd step (leaving 3hp over)
if you want the functionality of toolbox then this is one to go for as the last batch of them should be in stores now...
but... Pams should be able to cover the random into sample and hold functionality, as should (iirc) O&C with hemispheres...
I really like batumi and poti, but I'd be tempted to also consider zadar and nin - which can provide some very long complex modulation, which can easily be attenuated to mix with other modulation sources in the matrix mixer... I'd probably go with this and start saving for the next case - to put the other modules in - another very interesting module I'd probably look at is the Xaoc Samara ii - which has an interesting clamp function...
a good habit when you get a case is to charge rack space (ie save the cost of the hp for the module in a piggy bank or savings account) - by the time the case is full there'll be enough cash for the next case...
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities