How much can you get the D50 for in Sydney? This is probably the last chance before you jump, but if it's not so much different from D70s, get the D70s IMO.
About UV filter:
I think your 18-135 comes with a lens hood right? Just use the hood everytime you take picture, don't worry so much about UV filter, waste of money IMO.
Last time I was considering to use it because I didn't have the lens cap that could fit into my hood, plus what I have is Nikon L37c filter, which is not cheap at all, that's why I was considering to use it.
But now since I already found a hood and lens cap to go with it, I'm not using the filter anymore.
About 50mm, again, try to get a retractable rubber lens hood for it, so you also don't need to use UV filter. You can buy 52mm retractable rubber lens hood (for 50mm 1.8) everywhere and they're cheap. If you camera falls lens firts on the ground, if the hood is on, the rubber hood will take the hit, therefore preventing a dent on your lens (hopefully)
If you worry about dust on your lens, instead of buying UV filter, buy a Giotto lens cleaning pack for AUS$ 29.95. It comes with the rocket blower, microfibre cloth, the solution to clean lens, and 10X pointy cotton swabs.
The blower can be used to blow your sensor too if it got dust inside.
If you always use the hood on, and not overly clumsy, I think you won't even need a lenspen and can just use the blower to clean the occassional dust spots on your lenses.
PS: If you're planning to buy lens hood, or filter or whatever and wanna try it on your camera first, please don't let the shopkeeper try it for you. Refer to my earlier post. I speak from experience
PS #2: I forgot to say, since your dad is involved, why don't you persuade him to buy a D80 instead (ask him to chip in or something)? If you're having a hard time persuading him, you can use this quick method: (provided he's a camera guy)
1. Easy way to do is bring him to the shop that has both D50 and D80 on display (on the way there, mention how glad you are to have such a nice daddy), also bring your 50mm lens, mount on the D50 first, let him play with it (look through the viewfinder, the LCD screen on the back,etc..), and then mount your 50mm to D80, and let him look through the viewfinder. By this time he's probably realised how different the viewfinder and the LCD screen are.
2. Remove lens, put it back to D50 again, and ask him to take some pictures using different apertures. You'll notice that he will frame, shoot, put down the camera, look at the top LCD screen, and change the aperture by holding one finger on the tiny, semi-recessed EXP comp. button and rotating the back dial at the same time, and bring the camera up to his face again, and take another picture, and then for next aperture, same thing again. By this time he also gets a reminder that D50's viewfinder is small.
Put the 50mm back to D80 again, ask him to take pictures with different apertures. When he looks through the camera, he'll notice that again, it's freakin' bigger and everything looks clearer, and guess what, when he wanna take pictures with different aperture, he doesn't even need to put down the camera to check aperture, he can just keep sticking his eye on the viewfinder, and use his finger to rotate the front wheel to change aperture. This way he doesn't need to lose the composition and recompose.
3. Ask him to take pictures with ISO 100 and ISO 3200 on D50.
Well, apart from those 3, study the other differences between the two and overload his brains with all the cons. I'm sure he will let you buy D80.