I'm NOT an all around construction specialist, roofing being my main expertise. But I think the exterior walls should defininately be sided prior to the the Hardi-plank installation.
Over the years I've seen houses being built which went from:
to plywood on the wall corners and fiberboard throughout the rest of it,
to fiberboard everywhere,
to silver-faced foam everywhere,
to who-knows what everywhere these days.
I live in a 76 year old house with 2by4 rafters, no insulation in the walls, etc. When dealing with an older house I think one usually needs to compromise between what one wants, what one can afford, and what the legal/local building code requirements are. We all can't spend $200,000 and up on recreating our homes like they do so frequently on TV shows.
Contact the manufacturer, through their website, phone calls, etc. I'm sure they must have various "spec" sheets and/or requirements decribed somewhere for your perusal.
One other thing, I used to see felt paper used
on the "sub-wall" prior to installation of lap siding. NOt sure why this isn't done much
anymore, but I would look into it to see iof there are advantages to doing it. One thing that IS very important is to install flashings above all windows and doors prior to lap siding installation. I've seen many houses these days which have none. This means that caulking is relied upon to keep water from running behind them. The flashings are very easy to cut and install and are cheap to buy.
One who enjoys helping with roof problems, for the fun of it.