I'd also suggest drilling the tank, thats the best way to go. BUT if you cant or dont want to then you are left with deiciding if you want a sump or not. If you do, then you will need a hang on overflow. I used the cpr one as it uses a small air pump to keep the siphon going thereby mostly eliminating the potential of a flood. I ran that setup for several years and never had a flood, even though I have power outages all the time. Bout $20 bucks extra and maybe $15 a year on those aqualifter air pumps, they dont last very long but they work very well.
You can also bypass the sump idea and go with either a very large hang on filter, like an aquaclear 110. Plenty big enough and you can customize what you want to run as far as chemical media as it will fit fine inside. Easy to clean pretty much too.
Then theres the canister filter. I ran a fluval 305 on my 29g and then on my 55g for about a year and had zero nitrates and no issues what soever. BUT BUT BUT you must keep it clean! Nitrates will build up inside and no matter how many water changes you do it will never go down. So maintenance on a canister can be a pain, but it can and does work fine, even on a reef tank.