Is the duplex second hand or a new build?
If second hand, you may be able to get a lot of items left behind, ie kitchen appliances, blinds etc - be clear in what you're asking for while negotiating the sale.
If its new, you will probably have to set aside some funds for all of the above.
I know a lot of friends who went all out when they bought their houses, interior designers, custom items, the works! But this is not necessary.
When I bought my first property I did spend a bomb (new kitchen, flooring, painting and soundproofing plus all the furnature - cost a LOT) but I focused on the areas that I'd be using from day one, my bedroom and the living room for example and didnt worry about furnishing the spare rooms for some time. Also the previous owner left behind an old oak table and chairs and I ended up using them for about a year. They didnt go with my decor but they did a job, and I eventually donanted them to charity once I found a new set I was happy to buy which took ages. My mam was looking to get rid of a double bed from her house so I got that from her for the cost of hiring a white van man to deliver it, and ultimately we turned the 3rd bedroom into a home office so I was glad I didnt rush out to buy a bed for that room.
In hindsight, I definitly ended up with some items that I didnt love, probably because I was making decisions so quickly and without being able to see items together, so don't be afraid to take your time and don't let anyone rush you.
I definitely learned when I moved into my 2nd home. I took my bed and dining set, but bought new sofa as the living room is very different. We've done this one more slowly and took some furniture from the old place where it was still suitable. I still need lots of things for this place in we're in roughly 6 months. I've nothing in the hallway for example because I've not seen anything suitable and its not causing any problems so I'm happy to wait.
You have to prioritise - don't worry about spare rooms or side tables etc. Get yourself a good bed and a good sofa. Scrounge from family and friends in the short term or buy second hand if you can't afford to buy the pieces you really want just yet. Cheap sofas or beds are a false economy so do buy good quality items, and make do with hand me downs or 2nd hand until you can afford to do so.
Don't feel the pressure to do more than you can afford. Make sure you have enough to buy good quality essential items but don't worry about the finishing touches.