UV filters run water around an incased light bulb, this means the unit has to be kept dry as it's electrical, so in your case yes, install the UV unit on the discharge side of the pump. I understand they're water resistant, but I've always had an electrician install a GFI, used a drip loop and made sure the unit itself is sheltered from the elements. In theory it's best to install the UV after the mechanical and biological filter. If down the road you plan on building a pond with a bottom drain, waterfall/filter combo you'll have to install the UV inbetween the external pump and the filter box.
To answer you question about Koi, I wouldn't recommend them for a smaller pond like yours as they will quickly outgrow your pond; but they'll never make it to any size as I'm sure they'll die from poor water quality long before that.
If you want nice looking fish that will get a little bigger why don't you try "butterfly koi", a comet goldfish/koi hybrid. Eventually they may outgrow your pond but you may be able to keep a few for a couple of years.
Although I haven't personally dealt with Aquascapes, I've seen their units at local home and garden shows and thought they did a great job. I've heard they provide exceptional customer service so if you're only going to install a few ponds here and there you might want to give them a call.
Feel free to e-mail me with any questions, as it's not a hassle.