When I am acting for new clients, I always assess whether there is an immediate need for any changes of circumstances (such as relocation) or essential personal expenditure. This can usually be paid for in the couple of months it may take to finalise the IVA proposals, or perhaps covered by lower IVA payments in the first few months.
Clearly the dental work you require (so long as it is not cosmetic dentistry) is pretty important, and should be prioritised. Try and find a dentist who is still registered with the NHS, as this will save a considerable amount of money. With regard to new furniture, you can get some fantastic deals on e-bay and also local second hand shops.
If you have been sensible with your spending prior to the proposal of an IVA, and that can be justified, then it is unlikely to cause any difficulties for you or your IP, but as Philip has already said you should not use any further credit.
Regards, Melanie Giles, Insolvency Practitioner