Although you had useful replies from my mates Dom, Nick & Mike (who have all sailed with me on A89) here is my take on it, in the order that points have been raised.
I spoke to the seller a few days ago and he is expecting a signed letter from the owner, confirming free transfer of ownership. So on that basis, I would keep bidding and ask the seller to produce the signed letter before I went to collect the boat.
The missing parts – Secondhand sails are available and often come up on various websites. If anyone reading this has spare Atalanta sails, please post details. The other missing parts could be made, and we have a couple of members who “store” lots of spares.
I like your idea about budget dry storage – a small movable field shelter that can be rolled on and off to work on the boat.
Whether you decide to rescue A55 or not, best wishes.