Wow, I wish my wife would force me to buy camera kit!
Definately get a 35mm f1.8, you won't regret it and may find it's on the camera more than the 18-55. You may find you only use the 18-55 for the 18-24 ish range. I use a 10-24, 35, 50, 80-200 and don't feel I am missing any focal lengths.
I would suggest considering hiring (or buying if your budget stretches far enough) a telephoto for head+shoulders shot portraits of one or two people. Longer lenses, 100mm+ are much more flattering and if you get a fast one can soften the background nicely. Something like the 70-200 f4 would be a good choice (the f2.8 version would be nicer but a bit heavy for travel(?) and more expensive).
Flash wise, I'd go for a Nikon SB-400 for just over £100. The only reasons I have a bigger flash is to allow tilt and swivel rather than just tilt, and to allow off camera operation with my camera's flash commander mode which I don't think the d5300 has.
As for camera settings:
I think your biggest problem will be the overly bright sun creating large contrast giving you over exposed bright areas and/or under exposed shadow. There are some good tips here
http://neilvn.com/tangents/photograp...arsh-sunlight/
Assuming you are getting your camera kit in plenty of time, I would spend time learning how to
* use the different metering options so you can correctly expose for the subject rather than the background.
* use the flash to fill in shadows when the sun is behind the subject.
Avoid the sun falling across the face as this gives nasty looking shadows.
As the light fades, avoid using the flash as much as possible, they tend to give un-natural images unless used well, use the 35mm at f1.8 and as high ISO as you think your camera can manage without getting too noisy. Most modern digital SLR's will go to 1600 no problem. As long as you hold steady and your subject doesn't move, you should get sharp (enough) shots at 1/40s. But go higher if you can, subjects tend to move a lot when they dance!
If you have to use the flash, bounce it off the ceiling or wall (preferably not a coloured one, but that is out of your control) to avoid the harsh direct light.
Another technique you may like is slow sync flash. The fast burst of light from the flash freezes motion at that moment but the slower shutter speed (experiment to find what works) blurs the movement. It is an interesting way to capture the atmosphere of a party.
http://www.craftsy.com/blog/2013/09/...ing-reception/