once married in the UK you need to apply for FLR(lasts 2 yrs) then you can apply for ILR but you must have passed the 'life in uk' test first

and once your wife has been in the uk 3yrs she can apply for citizenship, (thou there could be some changes regarding this soon)
£2k+ would help your visa app, i wouldn't start spending it until your g/f has her visa. then you can
there is no minimum figure you and your g/f need to have each month, but a figure of £101 wk has been used in appeal cases as a minimum, thats your monthly income - (rent and community charge) hopefully will leave you at least £101 a week ?
your right about the 7.5 people

also
Rented accommodation If the accommodation is privately rented, the sponsor should submit a letter from the landlord that confirms both the size of the property and his agreement that an additional occupant may take up residence there. The sponsor should also include a copy of the tenancy agreement, and the tenancy agreement should have at least six months remaining until its expiry.