It does not sound good news as presented - high oil consumption, left pipes sooting. The water level falling is suggesting a leak somewhere - rads or the water bottle in the engine bay would be the likely areas to investigate.
The fact that it has had a rebuild 2k mile back means nothing without seeing the invoice of both what was done and by who. A true rebuild will replace all 6 liners with pistons, preferably with Nikasil liners, examine the crankshaft and bearings and replace/regrind/replace as needed. New timing chains would be put in if anyone uses a thorough approach the likes of Hartech in Bolton England. Hartech also machine the water channels to improve cooling on Bank 2 (cylinders 4,5 and 6) and will measure many of the components and advise what else needs to be changed.
Was the sale private or through a dealer? If the latter you may have some warranty to fall back on and push the car back to the trader to sort out. If it was private, did you have a full Pre-Purchase Inspection with borescope to see and check for scoring in the bores?
Logistically, if you have had the car delivered to Cyprus,it does limit the options for you to take up the matter with the seller (private or trade).