The weather at the end of September supposed to be mild with max. 20-25°C temperature daytime, 10-15°C nighttime and low probability of rain.
Mosquitos might be present, thus you might bring some spray against them!
If you need a visa to travel to Hungary, it is your responsibility entirely to apply for it in due course so that it will be delivered to you in time.
In case you need an invitation letter for a visa, please provide your legal private domicile or professional address, passport number and the address of the consulate in Hungary where your visa application will be submitted. Notice that the LOC reserves the right to refuse issuing an invitation letter if there is doubt w.r.t. to the purpose of your visit.
The local currency of Hungary is forint, euro is not accepted, you need to convert what should be done at main cities, however you might use your credit card. Suggest to change at official change sites.