1. Personally, for me. No. i have a non DAB RCD510, and have thought about it, but for the extra cost it simpy isnt justifiable in my eyes. £££ for the RCD unit, £££ for the DAB aerial.
2. cant help you there
3. I think it is easy if you want just the one option, ie ipad or USB or aux, as they all connect to the same port on the stereo, it is wanting more than once of these, the price shoots up as you would need an extra interface.
4. only certain onces with the video module would be, mine isnt. You are talking aftermarket or RNS system here.
Also, going from the RCD300 to a 510 unit you will also need a normal aeriel adaptor as the connection is different on the stereo. (a two-up connector rather than 2x connectors).