Upgrade from Nikon d5100?

My Nikon d5100 Shutter broke about 2 weeks ago and will cost $300-400 AUD to fix. So I decided to replace my camera and was wondering what would be the best low cost NIKON DSLRs.
I'm a plane spotter so I need something fast and would like Wi-Fi and
I don't need any lens.

Assuming your lenses are all DX you should consider the D7000 or D7100. It might be more than you want to spend, but shop around. The new Nikons are coming (the D5 and D500) so the prices on the "older" models should come down, especially used ones.

Whether it's shutter speed, frames per second, maximum ISO, or shutter lag, all dSLRs are basically the same. There are differences but you should be able to work around them to get the same shot as you did with your old camera. Just concentrate on your budget and get the most expensive you can afford. If you are confident enough, you can venture into the secondhand market.