Ambitious Summer holiday plans - car hire problem!

[Bjam99]

So it's finally sunny (in London anyway!) so I've been thinking about summer holiday plans. One trip I've always wanted to do, is to drive from Austria to Athens via the Croatian coast. I don't think my car would make it that far so that leaves car hire. However I'm struggling to find anywhere that will provide a quote. I had already kinda thought that the one-way hire fee might actually make it prohibitively expensive but it seems none of the usual suspects will even quote for it. Any ideas?

Maybe It'll be cheaper to just buy a car, (if I went the other way, might get some bargains in Greece?) but its always slightly risky. I don't really fancy sticking my head under the bonnet for a few hours in deepest darkest Northern Albania.

I want to do:
Vienna > Graz > Maribor > Ljubjana > Zabgreb > Gospic > Split > Dubrovnic > Shkondra > Pogradec > Larissa > Athens