I would definately buy a another pump then, filters are not my thing really but it has to be able to take the flowrate that your fall wants otherwise you end up complicateing the plumbing.
The shop has probably advised you based of a filtering requirement, 1/2 a pond volume per hour is the recommended minimum, but your flowrate is likely to excced that required for filtration. Some filters include UV's to aid combatting algae, I cant comment, nor can I comment of makes of filter, the commercial filter I have is a hozelock something bought because it was on offer and cheaper than I could buy the components.
To get back to plumbing etc
Personally I dont like cables or pipes over the sides of the pond and I therefore do what I can to avoid this. This may mean none submersible pumps and the plumbing going through the pond wall which is probably a bit much for your size of pond. So you are left with a sumbersible.
As I said I think you want between 50 and 100 gph per inch of fall width, I would suggest you limit yourself to 6" of width otherwise the flowrate will get excessive for the pond.
So thats the first thing you need to decide, how wide do you want the falls, that sets the minimum flowrate. Again you need to surf the web to check my figures, and watch out for sites quoting US gph, 1 UK gal is equal to 1.2 US gal so US flowrates will be numerically 1.2 times greater than UK flowrates.
You then need to decide the height of the falls and the type of filter, ie upflow, down flow, horizontal flow, there are advantages and disadvantages to all and really you need to do you own reasearch on this but your choice has implications for the size of pump you will require. If you go for down flow the highest point in your system where water and air meet is likely to be the top of the filter.
The height of this point ABOVE THE POND SURFACE is the max actual lift that the pump has to supply, this will be above the top of the fall since, more or less by definition, the outlet from the filter is near the bottom of the filter chamber. If you use an upflow filter you will need to us an oneway valve to stop the contents of the filter draining back into the pond when the pump is switched off, there are alternatives to a one way valve but this post is getting long enough as it is, similar considerations may appliy to a horizontal filter.
Having established your flowrate and maximum actual lift you need to choose the pipe size, I assume this will be pond hose, the simplest thing to do is use the largest possible size that will fit the hose attachments of your pump but you havent choosen that yet so its a chicken and egg situation. Flow through pipes generates resistance to flow via friction etc and I outlined the way it works innthe previous post. This resistance adds to the pressure that the pump has to develop to drive a given flow rate, this added pressure is seen as added lift, if you like it adds to the height that the pump has lift the water, this is known as pipe losses. The sum of the actual lift and the pipe losses are known as the total lift, total head etc. The greater the total head a given pump has to supply the less that pump's flowrate will be.
At what I imagine you will want, around 300 gph, I suggest a minimum hose size of 1", 25mm, as this should keep the resistance, pipe losses, fairly low but as I said bigger would be better. The things that attach the hose to the pump and filter are called hose tails and these will frequently be stepped to accommodate several sizes of pipe, cut off any smaller unused hose tails.
As you can see the pump is probably the last thing you should buy.
I think Hozelock and Oase publish pipe loss charts for their hoses and will email or post them to you, I have one here but I have forgotten whose it is. Going by that at about 300 gph
20mm hose produces about 1.2" of pipe loss per foot of pipe
25mm " " about 1/3" " " " " " " "
32mm " " about 0.2" " " " " " " "
40mm " " about 0.1" " " " " " " "
So when you work out what length of pipe you want you can work out your total head and walk into the shop saying I what a pump that produces X gph at Y feet of head.
Also consider the power a pump consumes and surf the manufacturers website as these frequently give power consumptions and flowrate against total head information. Personally I wont buy pumps off the web even if I have to pay more locally because if any thing goes wrong returning the pump can do away with the cost saving.
Hopefully Sean will come on soon and be able to help you there, let us know how you get on, details please.
