I'll have a go at some of your questions.
* Intelligent LCD (Demmel, anyone experience?)
No, but I've been looking at the 4D Systems LCDs and they seen pretty nice.
Realtime Clock (for log & maintenance messageS)these are not in by default, right?
can be partially programmed so that you don't have to do all store/execute all graphics on the Arduino.
4DS LCDs can handle all the UI if you want to learn their language (much like C I think).
It is a touchscreen though, so you are probably paying more for something we don't really want.
4DS have versions w/o touch.
Many 4DS boards also have SD mass storage.
(I have nothing to do with 4DS, just been looking at their stuff lately
Anyone knows such displays with good old robust buttons?
I think ALL the displays with built-in buttons use pissy little tactile buttons. They are quite nice for most things but useless for what you want to do I would say. Buy some proper sealed buttons and run wires to your board.
I wonder if all those components(LCD, Logger, XBee) fit without problems on a Mega.
I would think so but don't know if that combination is available as a single shield. Have a look at shieldlist.org
getting a new one (same model) is usually not much of a problem. Would that also count for Microcontrollers,
There is a large Arduino community now and there's a good chance the Mega will still be around (it's been upgraded recently but the new one is compatable with the old one). As for the shields who knows.
The Arduinos are open source so in theory you can always make your own (or get someone to make one for you but that would cost) if you grab the files now.
You can afford to buy 2-3 spares I would think.
What's your time frame? I would think the PLC option will be up and running faster IF it has all the stuff you need AND you know how to program them or there is a forum to help.
(I'm designing a Mega-compatable board at the moment that will do all this I think, but no chance it will be ready for you