We get 3-4 liters per head... I believe the warranty is 1, and 2-3 is average.
You're in greece, so I don't know your cost. But a head is a consumable and costs $100 here...Ink + printhead is comparable to solvent...depending on the machine. There's 20+ main solvent printers out there. If you need to replace a solvent head, you're looking at 3-4K because you need a tech.. maybe more in greece. But solvent heads dont fail often, unless you get head strikes.
Ask the company who services their printer - Call them up and ask them all this information... USA prices will vary greatly compared to your prices. And if theres no local tech... I wouldn't touch it, when / if something breaks it'll be hell to get it repaired.
Most of the stuff your asking, no one on here will be of help... We don't know what prices are like in your country. You may not be able to get ink and have to fly it in, which would make it cost 5x as much for all we know... your best bet is to ask your local distributor these questions and see what they say.