Solid State and kept crashing, popping etc
I've never had that problem with my Mustang and have never seen anyone else report that sort of problem, so just because another amp once did that, don't let it put you off the Mustang.
VHT Special 6 Ultra
I've read good reviews about that, I was contemplating getting one myself! If you already have that as a low-power valve/tube amp, then I'm not sure the Marshall Class 5 adds anything else? I'd agree with other suggestions to go with the Mustang Floor: you can use that with your current amp, and/or (as you suggest) connect it to a PA.
If only the Mustang Floor had a looper
I suppose there's a limit to what Fender can (or want to) include at that price-point. You can get multi-fx units with built-in loopers. But they cost more, and the loopers are quite limited. If it were me, and a looper were important, I'd buy a separate dedicated looper that has longer recording times and more features than the wanna-be loopers within some multi-fx units.
all the SS modeling starts to sound the same
I'd disagree, at least with the Mustang's modelling. Modelling technology is improving all the time, and many people say that Fender's latest offering in the Mustangs is a lot better than many other manufacturers' products. Although obviously I realise that what matters is what it sounds like to your ears, rather than anybody elses.