As far as a DSB goes, everything I've read from experts suggests that a properly inhabited DSB in a fuge is probably the best addition you can make to a reef tank. I haven't heard any compelling argument against it.
One of the best pieces of advice I can give is, there are many ways to DIR in regards to reef aquarium set up. No one way is better then another as long as you understand the limitations of the system you choose. The end result is the same, some I have found easier then others.
What your goal of a DSB or fuge, or both? If you have a proper sand bed in your tank, de-nitrification can happen in a shallow bed. One of 1-2" of depth IME is more then enough to take care of nitrates. If you plan on a bare bottom tank, then using a remote sand bed is fine to help with nitrates but IME, it is better left to its own container then using the sump. Sounds from your last reply you are thinking of both a DSB and fuge, you could do this in a separate tank and have that flow back into the sump.
My main argument about a DSB is I have never had one not collect detritus, years go by and it builds up to a point where it leaches and algae starts growing. I have read many of Ron shimek's views on how to prevent this but simply do not buy it. I have just as much success eliminating nitrates with an in-tank sand bed that I clean small sections of every now and again.
I have also had much success, with bare bottom systems, with high flow and good skimming. Along with proper water changes, never had nitrate problems in those systems either.
I have also had success with prodibio run tans, and zeo run tanks. Both though can get more expensive to run, yet did very well for me. Prodibio was the easier to use of the two systems, for me at least.
As far as fuges go there can be some advantages, like if you are using it as a tank to let pods breed that will later flow back to the tank. I though personally do not use them. Macros in general can go sexual. Chaetomorpha will not, making it in my book the best algae to use in a fuge. It can be a way to use up nutrients, that would otherwise go to growing nuisance algae in a tank. I prefer though, instead of diverting those nutrients to another part of the system, just to eliminate them all together, using other methods.
I do not want to steer you away from any method you choose to utilize.
Remember, I'm pro-DIY and anti-parting-with-my-money-unnecessarily.
I do not like to part with my money either. There are definitely ways to DIY and save money in this hobby, as long as you are handy. I have seem some skimmer that were DIY that were excellent like Mojo's. Yet I have seen a lot of very poor designs that people wasted money on a DIY version, only to spend the money to buy a proper one later.
As far as your suggestion goes about glasscages.com
Like I said they have a very dismal track record and shouldnt be trusted. But they are the cheapest I have seen.
The last 2 marine tanks I kept, I seemed to lose POUNDS of salt a day via salt creep.
I run both my DT and sump with no cover and have very little salt creep, yet very high flow tanks. My salinity levels hardly every change, and with water changes no adjustments to salinity is needed. Maybe it was the way your set up was last time, but there should not be an issue with this.