To address your points in order:
1. You can't have DCC and DC control at the same time. There is nothing to stop you installing a double-throw switch to switch between DCC and DC, but beware of point decoders and other modules if you have them. I don't think they would like DC current fed from the DC controller. It would be best to isolate those too if you have them.
Using DCC, you can usually have one non-DCC loco on the track, controlled with address '0' by the DCC throttle controller.
2. Consists. The DCC controller has to gang the consist locos together so that it can control them with one address. Your consisted locos will then behave as one, called up by any controller on the network by the consist address.
3. BUS. Yes, it's nice to have one so that you may add modules for expansion. I We're talking started with one controller - I now have 5 all linked together and will probably add 5 more if my 'World Domination' plans work out.
4. Computer interface. Remember there will always be people who want this. That's the way it is. I personally think a model railway loco should be controlled with a cab control from the side of the table, but each to his own. I have a computer in my track room, but for the moment it's used to listen to Internet radio. Computers do take the hassle out of complicated and complex layouts. You can program routes, automate sequences and work to schedules that flash up at you - just like they do today in the real railway world. You can have your detailed loco database link up to your control system and using a feedback system, you could theoretically control it from anywhere in the house.
5. Brands. Hornby DCC system and it's XPRESSNET are hot topics right now. This is a communication BUS that is set out and designed to a specific protocol. Other companies' products that also use the XPRESSNET or XpressNET BUS can be theoretically linked together. Companies that use XpressNET include: Atlas, Lenz, ZTC Controls, Arnold, Roco and now Hornby.
Loconet is another type of communication protocol. Loconet and XpressNET are not compatible. Companies that use Loconet include: Digitrax, Uhlenbrock, Fleischmann and a few smaller manufacturers.