I will address the points as numbered, for ease of reference:
1) Stepchange run a fair IVA, although, in the past they have been known to pass cases on to other firms. This used to be Grant Thornton. I do not know if they still do this with GT's new incarnation, Aperture. They do operate nationwide. I would be inclined to look at the smaller, more personal firms, like, well, the number at the top of the page here, or Lisa Thomas, who posts here from Neville and Co. Cambridge McDuffy also have a very good reputation, but there are many more out there.
2) It would be a good idea to get the new account up and running while your credit score looks better ( that will crash when in the IVA). Avoid First Direct and HSBC. Many of us use the Co-operative Cashminder account.
3) There is no easy to find link as the industry wants you to state what you actually spend and will advise if it is above the guidelines. BeatMyDebt used to have a budget calculator which advises when a guideline was being exceeded. Food, toiletries and clean allowance in 2015 was £222 max per single adult, £354 for a couple and £138 for each additional adult ( these have not gone up much since then !). Popping allowances in a series of jars is a common method
4) Each adult will be expected to contribute their fair share towards the bills, in relation to their relative incomes. But this can be discussed with the IP and negotiated.
5) A credit purchase, once you have decided that you are insolvent will not be looked on well and that creditor may object, though unlikely that they will look at that much detail. Many of us lived on credit until the bitter end. But keep it reasonable.