I used to live there. Split is not a place for a holiday, but it is in the centre of teh Dalmatian coast which absolutely is a holiday destination!
Split is Croatia's 2nd largest city (behind the capital Zagreb). However it is really only a good-sized town. You go drive from outside to the centre in 10 minutes. Most of Split got built up during the socialist/ Tito era and its fairly drab, grey concrete blocks of flats.
However the old town mainly consists of the retirement palace which was built in 300AD for the Roman emperor Diocletian and its still a living/ shopping/ eating out place today. The streets are obviously too narrow for cars so its all pedestrianised with a maze of narrow streets and a few wider squares. The lynx advert "angels will fall" was shot in the streets at the edge of the palace and it gives you a flavour for what the old town is like []
Split is also the main port for the ferries which take you out to the islands, such as Brac (50 mins), Hvar (1hr 45) and Vis (3 hours?). Most people stay there or further down the coast in the Makarska area (half way between Split and Dubrovnik)
Personally I would only spend one day in Split, and another day in Trogir (25 minutes away on the other side of Split Airport and for the rest I would rather be on Hvar or Brac or in Primosten (which is on the mainland, about one hour north of Split).
The good news is that everyone under 50 speaks some English as they learn it at school from the age of 7. Eating out and drinking are generally cheap.
The beaches are all small stones, not sand and as a result the water is the clearest blue you will ever see!
You will probably pay for your accommodation in euros but everything there will be in kunas. We found the best thing to do was just to use our UK debit/ credit cards to withdraw money at ATMs as and when you need it. Or you could take sterling out with you and change it at one of the (many) exchange shops. Most chops take cards tehse days but its rare in a restaurant or konoba (i.e. bistro).
Hire your car direct from the car hire firm: Hertz, europcar etc. The broker's charge less but the economy in Croatia is a huge mess right now and car hire companies have not been able to get leasing deals for new cars which has caused a hire car shortage in the last couple of years. When the staff at the car hire place have more reservations than cars, they tend to favour the customers who came to them direct and that has left the broker customers stranded (because you really need a car to get around in Croatia!).