Depending on the time of year and what you plan to do during your trip to Hungary, you will need to include various essential items in your suitcase!
Basically, Hungary has a continental type climate and experiences generally temperate conditions. Winters are cold across the whole country, colder than in the UK in fact. It snows often in Hungary. By contrast, summers are very pleasant, indeed very hot! With Hungary having no direct access to the sea, temperatures can reach 40°C in the countryside.
However, temperatures are milder around Lake Balaton, which is often referred to as the 'Sea of Hungary'. Balaton is one of the country's essential places to visit. So trunks/a swimming costume will definitely come in useful!
That said, Hungary has plenty of shops of all types and kinds in both its capital, Budapest, and elsewhere. So there is really no need to overload yourself with too much luggage. You can buy everything you might need locally, and often at more reasonable prices than in the UK.