I see no problem with a Dwarf Lion as the only fish in a 29 gallon tank. If this is really what you plan to do, then a sump will simply be unnecessary. There is no reason to spend the cash.
For this very specific aquarium setup, I would actually recommend the CPR Bakpak. It has a built in skimmer, and biomedia following the skimmer. This design is nice for fish only systems without live rock, because it allows the skimmer to remove organic waste, and any waste which is not removed by the skimmer then passes through the biomedia for breakdown into Nitrate. This design is unique in the hobby because ALL water flows throught the protein skimmer BEFORE it comes in contact with the biomedia. As a result, Nitrate accumulation is very minimal and easily controlled with water changes.
Amazon.com: CPR BakPak II Protein Skimmer / Filter w pump: Kitchen & Dining
There is a model on eBay:
CPR Bakpak2 HOT Protein Skimmer/Bio Filter - NIB - eBay (item 350273569036 end time Dec-02-09 06:05:54 PST)