An aquatic system cannot be constructed or operated any better than it's initial design. This statement is inarguable fact, and yet, few of us treat projects as separate entities in a/the planning phase.
This is the time to seek out, gather, ruminate and discern all facts, ideas, methods and attitudes as to the most appropriate, best available materials and technology for your habitat. Ask everyone seemingly knowledgeable everything; and believe no one. I don't intend to be cynical here, but to emphasize that you are going to be party to a wide range of "opinions" on many topics. Take them all in, visit other's aquatic efforts, then, judge for yourself!
Become critical and systematically inclined. Take photos, make sketches, borrow plans. Check out the largest library available for pertinent literature. See their Bibliography/Further Reading Sections in the back. Get thyself to your local tropical fish, koi, goldfish and outdoor garden clubs for info-swap and commiseration with fellow wanna-be water gardeners.
If need and pocketbook allow, call in the so-called experts. There are landscape and waterscape architects, brand X designers, specialty contractors and gardeners who would love to see your project. If you want, let them take a look and present their views regarding what's possible and how to get what you want. These folks can be accessed through directories, hobby groups and industry affiliations; ask them for referral.
What else about design? Be picky. Do keep planning until you're 99.9% satisfied that what you have on paper is going to work and get you what you're aiming for in terms of sights and sounds aesthetically and functionally. In the meanwhile, content yourself with roughing out your excavation until you've got your act together design-wise.
Kinds of Water Effects:
Are you looking for something formal; or more naturalistic? Do you have certain types of rockwork in mind, or at least some ideas of how large, color, angular...? In helping others make up their minds and guiding other contractors in how to go about doing the same for their potential customers, I've reasoned that there are two main approaches to design. 1) Listen to what the owner is describing they want the feature to do (raise waterlilies, attract birds, make splashing noise like a small stream...), and present options to them for achieving those ends, or 2) Exactly build just what size, shape, texture, etc. the owner has pictured physically. You are encouraged to vastly consider both approaches, and settle on the first.
A budget should definitely be part of your design plans. Some expenses in building out a system can be a revelation. Most folks are oblivious to how much a ton, ten or more tons of rock really is; and how difficult it can be to move around or remove. Digging holes and removing the "spoils" or excavate, might be cheap or expensive, depending on how hard the ground is, where you can/have to put it, ability to bring in power equipment...
By itemizing the "steps to completion", assigning a tools, material and estimate of labor to each, you and/or your contracted help will be able to closely approximate the project's overall construction costs.
But don't stop there. You really want to know how much energy (and maybe water) the feature will take to operate, doing what you intend. Large pumps for waterfalls and streams cost big money to leave on all night and day. How much?
Also, what sort of "other" maintenance costs and time commitment are you looking at for the given design options in front of you? Will filter media need to be replaced? What of chemical costs, if any? Re-planting, fertilizing of plant stocks? Well-planned and built systems require little ongoing labor, but they do take some time and money to operate correctly. Know and weigh your options before committing to the improvement; maybe an automated re-fill mechanism, or a vacuuming attachment, et al. is worth investigating and installing.
It cannot be stressed enough that laying out structural, mechanical, plumbing, electrical and rockwork plans must be done before proceeding with actual construction. When you're ready to proceed, you'll know.
Anon. 1988. Landscape architects explain why design makes the difference. Landscape & Irrigation 4/88.
Hough, Dennis. 1991. Backyard natives; a how-to guide to creating the backyard ark. Freshwater & Marine Aquarium 11/91.
McGuirk, Steve Frogley. 1989. Designing for good maintenance. California Landscape Magazine 5/89.