I was looking at converting a hardcore downhill bike. The unconventional frame didn't allow for a battery to be mounted there so I started looking into alternatives. Because of this I understand the dilemmas of battery mounting options. It looks like you will be presented with battery mounting options by using the Xtracycle extension however so don't give up just yet. It might be very possible with a little creativity and enginuity.
Dolphin style battery packs have taken on a smaller profile this year and some slide into the mount from the side to help accommodate frame clearances. Other options would include flatter, wider battery packs designed to mount to a rack, or a plain battery designed to reside in a fire retardant bag.
I would definitely suggest a mid drive conversion. The 750 watt BBS02 is capable of some pretty steep hills, or even the new 1000 watt BBS HD if you want to climb a tree.
There are several sights out there for conversions. Personally I found the guys at Lectric Cycles to be very knowledgable, helpful, and responsive in my quest for information on converting a "non standard" frame. You are probably looking at spending in the area of $1500-$2000 for a good kit with a decent battery and doing the install yourself. Battery capacity will be a big factor in price fluctuation. You'll probably spend an additional $100 per amp/hr you add to a battery, but if your wife is going to be hauling two kids and/or groceries, and climbing hills on top of it, you'll most likely want a battery in the neighborhood of 14 amp/hr.
As far as "simple" is concerned, it depends if you are referring to the day to day operation being simple, or if the installation of the kit is simple. The operation of the kit is pretty simple where the installation is not so much. There are a lot of videos on YouTube that show the installation process as well as videos explaining the procedures performed by the rider.
Hope this helps.
