I just did that journey thus last summer. It’s far. I did pretty much nothing but drive and even then it was two whole days from the Croatian coast to Dover. Don’t drive unless there’s a really good reason. You can fly to Ljubljana, Trieste, Zagreb and Graz very cheaply
There may be border controls or checkpoints due to the refugee crisis. There are still poor bastards trekking through Slovenia en route to the north.
You can buy vignettes for Austria and Slovenia at loads of petrol stations.
Slovenia’s lovely but it can be surprisingly expensive.