Car rental pickup Budapest drop-off Kotor - Help!
 

It's probably the countries you are planning on driving through, especially since that rental agency from Hungary will have to travel to Montenegro to retrieve the car. And, they are not willing to drive the 791 km (nearly 500 miles) for that. You may have to adjust your plans to rent in Slovenia, and maybe even again in Croatia.

Even if you find an option to rent, I'd expect that one way drop off fee to be VERY high.

Kotor is your problem, not major enough a city. With Sixt, looks like you can drop off in Podgorica or Dubrovnik.

No idea about cost.

Across countries both in and out of the Euro and the EU. Not only does it make complete sense that it is difficult but the cost of repatriation would need to be included.

I'd find a broker like autoeurope, give them a call and let then negotiate for you, but expect your credit card to squeal.

Using public transportation for parts of your trip might be worth considering. As just one example, it's easy to get from Ljubliana to Zagreb by train or bus, so you could return one car in Slovenia and pick up another in Croatia.

I suppose the OP has either some health problems and can therefore not use public transport
or wants to visit many absolutely unknown places without any transport infrastrucure.

Along the main iitineraries llike
Budapest - Maribor - Ljubljana
Budapest - Lake Balaton - Zagreb
Ljubljana - Istria
Ljubljana - Rijeka
Zagreb - Sarajevo
Zagreb - Istria
Zagreb - Rijeka
Rijeka - Kvarner Islands
Rijeka - Zadar - Split -- Dubrovnik
Zagreb - Plitvice - Split - Dubrovnik
Dubrovnik - Hercegnovi - Kotor - Bar
Split - Mostar - Sarajevo
Split - Hvar - Korcula - Dubrovnik
Split - Vis/Brac/Hvar
etc.
public transport (mainly bus, resp ferry or train) is very good.

There are a lot of very good reasons to enjoy car travel, since you are not locked into going from point A to B. Stop anywhere you like, anytime you like. It's go at your own pace.

You are right, shelemm, of course.
But most posters in this forum will just not have the time for that.
Slow pace travel from Budapest via Ljubljana - Istria to Monenegro could easily take more than a month or two.

I think the OP's trip is doable and will be amazing. Evidently, some form of public transportation will be needed between the OP's two endpoints. If the OP wants to get off the beaten path during the trip, I understand the impulse..

Whether the OP's trip makes sense depends on what the OP wants to see and do -- and we don't know how much time the OP has or anything about the OP's interests. FWIW, I spent a month traveling between Ljulbljana and Kotor and definitely skpped a LOT ... of what would have interested ME. To each his/her own.

Most posters in this forum want to do too much and will not have time for any sidetrip.
Therefore, they end on the motorway in order to reach the next destination in time.
In this case, they could have travelled by bus or train as well.

If you are located in the US, I would call both Autoeurope and Kemwel and explain your plans to them and see what they say. A few years ago we drove from Croatia to Sarajevo and back. One of those companies would allow that trip and the other would not.

The one way drop off may be an issue but it is worth trying to see if you can navigate this. If not, you may need to consider dropping off cars in the same country where you rent them and taking a different form of transportation across the borders. We did that once, dropping off a car in France near the Italian border, taking a short train ride and then renting a second car in Italy.

It sounds like you have an amazing trip planned!

I see value in pointing to public transportation options, but do not see the value of generalizations about what most posters do or don't want. Even if this generalization is true about most posters on this forum -- and I have my doubts about that -- we don't know the OP's preferences, interests, or plans. I fail to see justification for proffering a pejorative interpretation.

So many generalizations.
The OP is proposing a car trip that is about 16 hours of driving. If that is over the course of two weeks, that allows for plenty of activity plus stops plus relaxation.

"The OP is proposing a car trip that is about 16 hours of driving"

Guardiamo un po:
Budpaest - Bled: 6 hrs non stop. Stops at Veszprem, Szekesfehervar, Lake Balaton, Varazdin, Ptuj and Maribor are musts
Bled - Divaca caves: 3 hrs non stop. Stops in Isonzo Valley, at Miramare, Triesteand at the sud of Lipica are musst
Divazza - Pola: 2 hrs non stop. Stops along the coast (Parenzo, Rovigno) are musts
Pola - Kvarner Islands - Plitvice: 8 hrs. The visit of the Islands is a must.
Plitvice - Zara: 4 hrs non stop
Zara - Sibenik/Skradin - Trogir - Split: 3 hrs non stop. Stops at Krka Falls park and at Trogir are musts
Split - Hvar Island - Mostar (with car ferries); 10 hrs. The visit of Hvar Island is a must.
Mostar - Kotor: 5 hrs non stop: A stop at Dubrovnik is a must.

This "classical itinerary" alone would give stuff for 2 weeks

I've done a fair few bits of those roads by bus, very easy and fun. But then again I get no pleasure from driving a car and get real pleasure from chating to locals.

Why don't we simply wait for the OP to chime in with any questions on their driving itinerary? Until then, we'd be speculating over possible details. The cybervoid only cares so much.

Thank you for everyone's input. We have decided on a different trip.

I was wrong, sorry! According to his new post, the OP wants to visit big cities and (over)touristy places only.