Hi Mikke, The grimeca kit is good, it must be as they supply the disc parts Vespa, and LML, use as standard. It's true the semi-hydraulic master cylinder is a weak point, I've seen some people that love em, and plenty that don't. I have heard of a few bad ones that don't work well, must've been a bad batch made.
What scooter do you have? If it's a PX then the easy way is to get the PX disc headset and just swap it over with yours.
Personally I think the PX master cylinder is too small, I don't like it.
I have been working on a full hydraulic system which uses the original throttle pulley and keeps things looking fairly standard.
If you want it to be sporty, then I'd recommend getting a quick action throttle tube, you can bolt a motorbike master cylinder to it and use a qa throttle.
I use 12.5mm bore Suzuki, and Yamaha master cylinders, I have found these to give an improved brake over the Vespa 11mm bore cylinder.
If you have a lathe you can turn your own throttle tube, I use 1 inch (25.4mm) H30 aluminium tube with a 1/2 inch bore (12.5mm).
Take a piece the length you want (measure your vespa's throttle tube and add a little to allow room for the master cylinder, and qa throttle, I generally add 1 - 2 inches (25.4 - 50.8mm ).
Turn it down to 15/16ths (24mm for a length, enough to fit in the headset), mark where it leaves the headset and turn the rest of the tube down to 7/8ths (22mm to fit the master cylinder, and qa throttle). Fit it in the headset and drill & tap the headset, and tube, somewhere so you fix the new tube in place.
Cut the lever pivot off the headset to make a space to fit the master into and fill the holes in the headset casting with weld, or liquid metal/filler (whatever you want, it's only to tidy the look of it).
I can send you some pics of a conversion I did to a Mk1 T5 headset, or look on Scooterboyworld, its there somewhere, or look for the thread with my LML thumb throttle conversion on here.