Sounds like you have three choices, no DSB, in tank DSB or remote DSB (fuge). If those are your choices I'd stick with remote DSB (RDSB). I did it that way and it worked fine for years.
With DSBs, the first two to three inches is overhead. Bacteria in that range convert waste to nitrates (the bad stuff). Between 4" and 18" the bacteria converts waste to nitrous oxide (the good stuff). Below 18" the not much happens. The more RDSB you have in the 4" - 18" range the better your good to bad ratio is.
In any case, if I had a choice, I'd install the RDSB so I could replace the sand in 25% parts without disturbing (introducing oxygenated water into) the rest. IME, as the sand gets around 5 years old it becomes nutrient saturated (unviable). Find some deep tupperware containers that fit snuggly in your fuge, line the bottom and fill them up. If you fill the containers/fuge with water first and poor the sand in slowly you won't trap any bubbles in the sand. This should help the good anerobic bacteria take hold faster.