Hiya Oneflewover, I write a blog about camping and bongos. I have explained lots about buying and converting them, people seem to find it useful! You might want to start here with 'So you are thinking about buying and Bongo' and see how you get on.
My initial thoughts on hearing the dimensions of your family is that your husband might find it a bit of a squash if you do a full side conversion or a half side conversion, as in adding sink, cupboards, fridge etc along the side . The bed becomes fairly narrow, my husband and I are average size (5ft 5 and 5 ft 11) and we no longer both sleep down there as we found it too squashy, I sleep 'upstairs' with one of the kids and he sleeps 'downstairs' with the other. This may be a solution for you too, though I know some people find it a bit odd not to sleep with their DP.
Alternatively you could think about leaving the cabin unconverted. The amazing thing about Bongos is that in their original state you can lay ALL the seats flat to create one mahoosive bed.
Another possibility would be a rear conversion, the kitchen etc is then accessed from the boot. You retain the width of the bed for sleeping, but you have to open the boot to use the kitchen, fine in good weather, a bit drafty in bad weather, but lots of people do choose rear conversions so its definitely a possibility.
The 'upstairs' sleeping arrangement is hard to explain but I will try. In the original set up there is a hatch in the middle of the roof that you have to climb through and then close to lie down on it. This is a tremendous faff if you have kids and if anyone needs to get up in the night. The answer is to have a conversion which means you can access the roof from a flap at the back of the roof (inside). When our kids were younger they both slept up there with heads towards the hatch and the hatch open, if they needed me in the night, I just stood up and I was right there. This may sound unsafe but when you see it you realise its fine, though you might want to put the small baby in a pop up travel cot tent or something. My youngest was 3 when we did this. I had this conversion done at Clearcut Conversions in Barnstable, I genuinely wouldn't consider using the Bongo with young children without this alteration.
As you may have gathered the best way to work out what is best for you is to go and look at different conversion. See the roof up etc. There are a few big importer/converters who will be able to show you different conversions, get to one and have a good nose!
Finally, I would say, we love our Bongo it really is a great alternative to a VW. Ours has been very reliable (just be careful when buying!) and we have had so many great camping breaks in it. Do have a look at my blog I talk about what to look out for when buying a bongo and how to import one. Good luck!