I built mine in 1992 for $72,000 which included the trailer. It is on a trailer and can filter 40 GPM's. She weighs in at 9,000 LBS and some change. Though I may have some photo's, I do not post them on the web.
I have thought about building them in the future. For 20k, you need to ask these questions...
1) what am I going to use it for ( shopping centers, fast food rest. garbage bins etc)
2) what size of a unit is best for me? GPM's?
3) How big of a trailer is needed?
4) Power requirements? Mine takes 15 KW diesel generator to run it.
5) Elements need to recharge the unit. ( Example... clay, polymers, cartridges etc.) Know how to discharge of them legally and the costs. Also how easy to replace them.
6) Know that a filter is designed to take particles out from the liquid. Know how much storage of these particles can the filter hold before it gets clog. If the filter says " Under ideal conditions" it will filter "up to" 20 GPM's, know that in a garage situation, that will last about 2 to 4 floors more or less before your GPM's will be cut in 1/2 to 3/4 of the flow volume. Meaning 20 GPM"S down to 15 GPM'S down to 10 GPM's down to 5 GPM's. THIS IS A REAL Possibility if the garage has allot of particles, brake dust, dirt, oil, etc.
7) When we do a garage, we can get UP TO 30 to 300 gallons of sludge. If your filter cannot hold that much, then where are you going to store it while on the job?
8) Trailer design is needed with a cad program for trailer fatigue. ( This is an option, but is well worth it ) we balance our trailer with all 4 wheels off the ground suspended by 4 corner 18k jacks. This way we can level the filter for proper filtration. Our trailer was designed to handle 6,250 lbs every 4 feet while suspended off the ground meaning that the trailer will not bend or sag while the wheels are off the ground. Our suspension though cannot handle that kind of weight. Our suspension is only 14 KW.
So I guess what I am saying, if you are planning on doing a Mc Donald's Drive Thru, get a Hydro Tek. Trailer design is not a big thing.
If you plan on a 40 GPM+ filtration unit and are thinking of doing 500,000 sq ft in 6 days, get some help.