i think you've about covered it mate to be honest,
if you're not going to pull the 146 motor down and preserve the parts seperately, drain, cheap oil and flush, refill with oil again, and very importantly, dry store and turn it over reguarly.
the most damage i've seen in static boxer engines is when the oil is allowed to settle at the bottom of the cylinder for too long, and this has led to the beginnings of a seizure from drying out and gumming up. your fresh oil following a flush should really help this, but old oil can become quite acidic too and cause pits in the cylinder when left static.
cheapest places i know for 16v gasket sets are unipart motor factors, or rockauto's.com have been the most competitive recently, £100 ish for a FULL gasket set.
good luck mate