Hi Jen - good choice. We upgraded last year from a 4m to a 5m and have not regretted it - loads more space. I am not sure anyone could tell you which brand is best - Soulpad, Belltent.co.uk and Obelink all sell tents made of the same grade cotton - Belltent's and Obelinks are the same tent - loads on here about that.
Then there is Karma Canvas and Canvas and Cast and other online sellers who use heavier grade canvas or have other extra features like mosquito nets over the doors or different coloured bells (like Boutique Camping).
I don't think anyone who has had an obelink tent has ever complained about it on here - we certainly love ours - and they are significanlty cheaper than the UK sellers at the lower end of the price range, ie. the tents with separate groundsheets, which is what we have.
Back to your questions, the deluxe has a sewn in groundsheet and the ultimate a zipped in one, so you can zip it off and roll up the tent sides in hot weather. The price difference between Obelink and Soulpad starts to drop off with those two tents.
If you want a proper bedroom in the tent you will need to buy an inner, again a lot cheaper from Obelink. However, Soulpad do a quarter size inner, which I don't think any of the other sellers do. Bear in mind, inners do cut down the feeling of space which can be a factor against them.
Other accessories - coir mat etc. are nice extras but aren't essential by any means. It is always nice to have some throws and cushions, bunting etc. for a bit of camping glamour but part of the fun is finding your own rather than just going to belltent.co.uk or wherever and buying the whole kit from them (just my opinion obviously).
As far as full height airbeds are concerned, Aerobed are apparently the kings of airbeds generally and come with a price tag to match. We have an Intex which is good and I know they do a double height version, although we don't have it. We do have a superking airbed though and there is plenty of room in a 5m for it. I think Intex are American but there are online sellers over here.
Happy Bell-tenting!