Hi
A HIP pack will definately be required before the estate agents market your property, you will find places online that do them for about £300.
Solicitors do not need to be instructed at this stage (they do not need to get involved until you have accepted an offer) so you shouldnt have to pay anything to them yet. Also you're best to ring around solicitors and get prices anyway because the ones that the Estate Agents recommend are usually more expensive, I expect the Estate Agents is getting their cut!
It may be worth finding out how long a HIP pack is valid for and then at least if you do have it done now and cant get a new mortgage at the moment maybe in the near future when the 'credit crunch' has calmed down youl be able to get a new mortgage and re-use the HIP pack?
I think HIP packs are an absolute con!!!