I need an interface kit to make a professional GUI for a Pd patch.

The kit will need to feature drop down menus, sliders, text/number displays and toggle switches.

I dont really need fancy graphics, just a solid basic interface more conventional that possible with Pd objects.

I'm using Linux, dont really need compatibility with other platforms.

GrIPD does this: http://crca.ucsd.edu/~jsarlo/gripd/

yeah I've had a look at that before, no support for drop down menus tho

have a look at http://www.processing.org

Processing ? Using Java for my beloved GUI ? NEVER :p
Gripd would be really nice is someone could carry on its development. It's not stable at all with my ubuntu 12.04, but i really like the way it should work, fast and simple One could use python+gtk+osc and achieve the same results, but it would be longer and more difficult.
Have a look at pyGame and inkface (SVG for GUIs ! Sadly the development looks stopped since 2009)