There are some designers in my area that charge a flat rate per hour for a general consultation, and will provide recommendations on materials, colors, finishes, etc. Of course, they are also more than happy for you to hand the entire project over to them, as well. I would ask around in your area.

If you are doing any structural work to achieve your open concept dream, you need to have an estimate that reflects this. You may need a structural engineer. Having a designer draw up plans and choose furnishings before the scope is determined will be a waste of $$. Before you hire a designer, hire a contractor who can help you determine the scope. Then, when you understand your budget, hire a designer.

Then yes, a designer would be helpful. I'm not sure what area you're in, but I live in San Francisco where a designer costs around $100-180 per hour. You can also try Decorist or other e-styling services, but a lot of those are focused on helping select furniture. If you want someone to help make your space more cohesive, I would suggest calling a few designers in your area or, if you live in a city, find a furniture retailer who carries items that you like the style of. They may offer free design services and input, though this will not carry over into wall colors or anything not related to furnishings.

Give an on-line service called Havenly a try. They have three price packages, with the cheapest being $79, and a lot of decorators to choose from, depending on the style you're interested in. They have a chat room, which is free, where you can tell them exactly what you need, and they'll get back to you with a lot of information. When I chatted, the designer made a recommendation for the living room, which was rather minor, but it opened up the room more and inspired me to change a few things in two bedrooms which turned out very well. It didn't cost me a penny!