After you have assessed the spacing and number of spindles you can fix hand rail , offer in all the spindles then you can move them along one at a time and add your spacers ,
A couple of Inverted L shaped supports would allow you to place temporary posts outside the footprint of the shed allow easier access to replace everything.
£100 for a bottle ? Refills are £30-40 and you can get a bottle for a few £ from Facebook marketplace or even free , I recently picked up three bottles for free and all had gas in them.
Be aware some brands of gas use a different size connection .