I'd be looking at the Digitech RP500 or RP1000 then (caveat - I've never played one), but I've owned a M13, Nova System, HD500 and now have a G-System (amazing unit, but it doesn't suit your needs). I say Digitech because they get favourable reviews over the HD500, and it seems the only benefit of a HDx is the amp modelling, and since you don't need that...
I'd go with the RP1000, the extra footswitches (for me) would be worth it, so you can use patches and also switch fx on and off. For me, only having 5 switches on the RP500 wouldn't be enough, but I like to use only 1 patch live (my setup on the HD500 was 3 different kinds of overdrive/distortion, one for the wah, tremolo, delay, gate, boost/EQ).
If you don't mind an external expression pedal (have a look at the Mission Engineering ones) then totally try a Nova System. As Les said, the time based fx are top notch, and it has an analog drive section. I found "only" having 1 overdrive and 1 distortion was limiting for me (as per above live setup) but it might be enough for you.
As always, try before you buy, dedicate a day to going into a music shop or 2 and trying the units out. Take your guitar and amp in and see how they all work together (if possible).