for a guitar input I really like the befaco instrument interface - clean (unlike the doepfer) and has envelope follower and gate extactor built in... and can take mics (including phantom power) as well, so really interesting addition - to quote Ed O'Brien "guitars are oscillators"
maths is almost a no brainer to me - download the 'maths illustrated supplement' and work through it a few times concentrating on what, why and how - it will massively improve your patching game!
modulation is always a good addition - as are utilities - things that can route signals about in different ways and alter them - they also have the advantge of tending towards the less expensive side... if you ask me these are probably the most important modules in a modular to have lots of... see my signature and have a good long think about it & how it would apply to your
if you're a professional guitarist - I expect you have at least a few effects pedals about - get a pedal interface or 2 or 3 and try patching them in - experiment with delays and reverb before filtering etc
"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