1) Is it that much better than the ME-20 for instance (will the extra cash be justified)
Yes, Not only does it have more features/FX, but it has better D/A converters, which give it a better sound quality overall.
2) In other posts Alan says that the ME-70 would be a waste played through an MG Amp...
Some folk like the MG tone though - so make up your own mind. I'm quite honestly a bit biased as not a fan of the Marshall tone at the best of times (although I do understand what the attraction is in an 18 Watter or a JCM 800), but the MG is (to me) not even a good realisation of that tone.
The amp is the heart of your tone and I believe you should be happy with it just plugged straight in. Any effects are then used to broaden the palette of the basic good sound provided by the amp. So if you don't like the tone plugged straight in, you have the wrong amp and are probably using the FX to compensate, which is never a good idea. A good amp will sound good regardless of what type of pedals you stick in front of it - stomps or multieffects.
3) Would it not be better to rather buy a decent amp with built in effects...?
The really good amps tend to be sans FX. Some of better solid state stuff like the Vox ADs and the Line 6 amps have FX too, but that does lock you in to what they give you - a separate amp and effect setup gives you more of a choice in what make/models you use.
I have never really been happy with my tones & am seeking something that can help me obtain the exact sounds I like.
What is that tone? Or a good example of something similar.