Hiring a consultant is always a good option provided you hire the right consultant!!! (after all Consulting is my profession though its in IT). Apart from the points Venkat rightly put, a consultant will help you focus on the important aspects and will provide the best way of creating a system that mates with each other.
I would rather ensure the dedicated room is acoustically well designed than buying a mega monster amp (fancy brand), bcos I can always buy a better amp later but re-designing a room is messy, costly, and totally nerve exploding experience.
If you can find a practical consultant who is knowledgeable you can hire him. You can always ask HFV if the suggestions made by him/her is good.
I will stay away from any consultant who says "2% of equipment cost" no I will run away...90% he will sell what you dont need.
I would prefer someone who comes home, understands your need, and then says xxxxx is my charge for this and once we design it, you can buy and give me or I can help you buy them!
A consultant is like an architect, the decision is also similar and so are the headaches!