1) No. Alphards have three rows of seats. Front, middle and rear. Some Alphards have captains seats in the middle row, some have a bulky fixed armrest structure which, I imagine, will impede sleeping significantly, The rear seats will fold flat, or are moveable to sit alongside the rear sides. Many Alphards are able to accommodate a double air bed.
2) You do not mention a budget. Post 2008 (known as a Gen 2) seems a good choice (beware the Captains seats where the armrests are fixed). Some have middle seat entertainment to the roof; be cautious if electing to change the head unit. Many work well, some don't.
3) Hybrids can be very expensive to rectify, but reports regarding the Gen 2 versions are much better then the earlier models.
4) A 2008 will be ULEZ compliant. You can check the registration on Transport for London website.
6) Personally, I would recommend a report by the AA or RAC. I had one, very thorough. They are not cheap, and will detect many potential issues. Even if no issues are found it is very reassuring. Also ensure you are given a copy of the Auction Certificate BEFORE you place a deposit or pay in full.
7) Ensure your purchase is undersealed