Lovingretirement,
Try http://www.bom.gov.au/climate/averages/tables/cw_014015.shtml for Darwin climate info.
Jabiru http://www.bom.gov.au/climate/averages/tables/cw_014198.shtml
Timber Creek http://www.bom.gov.au/climate/averages/tables/cw_014850.shtml
Broome http://www.bom.gov.au/climate/averages/tables/cw_003002.shtml
This way you can decide for yourself. Comfort depends on a number of factors and we each respond differently.

I would not aim for right at the end of the dry because of my interest in wildlife. August and September would be prime from my point of view.

I'll let someone else deal with the car issue but suggest you contact on of the places on the Gibb River Road as they will know which company has cars coming through.

If you are from Miami then 6 months of the year weather wise up there in W.A NW would be fine.
Personally I prefer the wet, or after the wet, because all the waterfalls are flowing and there is new life everywhere but that depends on what time of year you could manage to get there.
I was not taken with the car hire in Darwin the last time we were there as they wanted to impose dreadful excess charges so do check that out first.
I presume you are talking about El Questro Station when you talk about the B&B up there on the Gibb River Road, if not then check that place out http://www.elquestro.com.au/ Write to them asking their views on which car Company etc they would recommend and anything else that you need to know about the area. They should know the best time of year and what, where, and how to do that area and I am sure that they would only be too happy to give you the best of all information.
7 days should be good to get from Darwin doing some side trips en route.
I do know that El Questro has a lot of different accommodation available as well as tours, side trips etc in that amazing area and you may find some adventures that you had not thought about.

ALANJG and MaryD2: thanks so much for your input. I know accommodations in these areas are limited so if I plan on going this year (Aug/Sept/Oct) I better get moving. I already preliminarily looked at the lodging offered by APT in the Bungle Bungles (Wildnerness Lodge) and it looks like May thru Sept already unavailable. I will call them directly to ensure online info is accurate. Mary: did you stay at the El Questro Station? If so, just wondering your opinion of the difference between the bungalow's and the homestead rooms. Also, sorry but by BB I meant the Bungle Bungles. Because of the one way excess charges, might have to look at two separate bookings (one out of Darwin and the other out of Broome or Kununurra and then flying in between. My bigger concern though is what areas the rental company will not permit the cars to enter.

There you are now, I forgot on how many things could be BB! I personally have not stayed at El Questro but in-laws went there about 2 years ago and a still raving about it, but then you can never shut up inlaws anyway.
From a car rental point of view I think I would try and fly in between some of the points too actually. Getting a car in Darwin and doing that area then, as you say, getting another vehicle in Broom, but check out that availability ASAP. The issue about driving hire cars into the Bungle Bungles is why I suggested writing to El Questro and putting that question to them OR directly to the hire car company.