Octopuses must ship next day air. If you have the option, arrange to pick it up at the FedEx/UPS facility and arrive when they open. You will want a shipper that will ship with excess water and preferably a styrofoam cooler and heat or cold packs to approximate 72-78 degrees (cooler probably better than warmer but not cold). Shipping keeps going up so my estimate of $60 for the animal and $35 for shipping may be low.
Inking during shipment is almost always lethal but, fortunately, not common. Acclimation is usually about 3 hours. If it inked and is alive, create a new batch of water as close to the shipping water as you can at a very rapid pace and get it out of the bad water, then acclimate to your tank.
If you find one in an LFS, make them let you see it eat.
I did not find a skimmer necessary with the mercs (but do with the larger animals) and it is unlikely you will find one that does much of anything for that sized tank. I would place your rocks in the center, to discourage climbing into the filter. However, you will need a lot of CO2/Oxygen exchange and a decent filter arrangement My very old, original style Nano has miserable passive filtration and I would encourage seeing what you can find in a cascade filter to fit in the back and dump into the tank.
Create red lighting. Mercs are nocturnal (IME, more crepuscular and can be trained to eat around 11:00 PM maybe earlier with room lights out for a couple of hours) but do well with red lighting. I leave mine on 24/7 (on during the day to avoid adding a timer, on all night to avoid a time of total darkness and acclimate them to dark being red light on). You can use any daylights (or none at all) that come with the unit but you will want to turn them off several hours before feeding.
Mercs are a "large egg" species. If you should get an adult female, she won't live long but will likely have mated and it is possible to raise the young. We have had a couple of members mate the siblings and raise the grandchildren. Again, not simple but doable.
In a 29 you can keep a pair and I think they are more active when kept this way. However, anecdotally, the pair should have been found together and or be siblings. The best source would be from a live rock farmer that found them as bycatch in the rock.
If you can go up 10 more gallons you could keep an aculeatus. The tank would be on the small side of minimum and you would need a skimmer but aculeatus are diurnal and, when you are lucky, a more active species. These most often come from Live Aquaria, have an arrive alive guarantee and are always packaged well. Unfortunately, they are also often full adults and are a small egg species so there is no chance of raising offspring. Here is a link to our most recent journaled addition,
Kobe. If you subscribe to the journals and set your profile to allow email, you can follow along with his journal without having to remember to look.
For journals on the mercs (as well as other aculeatus), you can use the search function and look for
mercatoris in the title or use your browser's Find to locate mercatoris in the
List of our Octopuses 2013 and
List of our Octopuses 2003 -2012 . From 2008 forward, the name of the animal will be a link to the journal.