Given your space restrictions you may want to go with a pre-formed liner and a simple drop-in pump.
Preformed Plastic Ponds - Everything-Ponds.com
Pondmaster Starter Pond Kit - Includes Pump, Filter and Fountain Head - PMK190
I put in a 2500 gallon, 40" deep pond with skimmer, particle filter, and bio filter about 10 years ago. It is NOT a set it an forget it project. Every week there is something to do - usually cleaning the particle filter. I have to keep herons away because they LOVE colorful fish. But it is amusing and there is always something going on. Right now the green frogs are mating. In a few weeks the bull frogs will be at it. Then the fish will start mating. We have some golden orfes (fish) about a foot long that prowl the pond like sharks and jump halfway out of the water to grab something to eat. It is very relaxing to sit out by the pond with a martini, listen to the waterfall, and watch the fish glide along.
The pond is seasonal here in Ohio. None of the water lilys are up yet and the ferns that grow in the bio filter (black thing at the top of the waterfall) have not come up yet to hide it so it looks kind of bare and ugly at this time of year.
There are at least 12 green frogs mating in the pond. This little guy is looking for love. We'll probably have a couple thousand tadpoles in 10 days or so.