Maths is never going to be a bad choice - especially if you download and work through the (imo) number 1 learning resource in modular... the 'maths illustrated supplement' - I see Falistri and I see cramped and confusing - but some people seem to love it, each to their own..
not that I would describe either as utilities - they may include things that could be seen as utilities, but should be seen as their own thing and not 'utilities' as such
I always recommend a utility starter set (these are modules that are always going to be useful): links (perhaps not so useful in an effects rack), kinks (discontinued - buy if available, else substitute wmd/ssf toolbox), shades (or similar - multiples of this will be useful without links) and veils - or modules that cover these functionally - nb get a quad cascading vca such as veils or the intellijel version or the triple one from happy nerding - but read the specs and understand the differences before buying!!!
I'd look at the FX Aid XL as an additional effect and a matrix mixer (doepfer make a reasonably priced one) as being incredibly useful for setting up send/returns, mixing modulation sources to derive more interesting ones and creating feedback loops
Utility modules are the inexpensive, dull polish that makes the expensive, shiny modules actually shine!!!