Eheim don't make that model any more.
Its roughly the equivalent to a fluval fx4 or eheim pro3 2080, so if it where new the price would be about right, but its a discontinued model so i would say €350 is a bit pricey considering you can get a current model equivalent for about the same money. Or you could look at it that you found a great filter when there arent many of that model around any more. You havent been ripped off though, as long as its operational, has been well maintained and had parts replaced to a reasonable schedule. If it came filled with filter media, id say you got a great deal. It looks like you can still get spare parts fairly easily, so thats good.
If you contact eheim i bet they could email you the manual, but its rated for 265 gallons i think and i would trust eheim to be honest about their rating more than say fluval.
How often to undertake maintenance is situational. I like to give the media a bit of a rinse monthly on a canister, but usually actually get round to it every 2 or 3 months. Really depends on how much detritus it is pulling in, and how quickly the sponges get clogged up.