If you go for 2nd hand canister filters, the Eheim ones are usually worth buying, they are tried and tested and have a reputation for long term reliability (still have to double-check the working condition before buying though). Look for those units that are only used for a few months with receipt, they are almost as good as new.
Personally, i've also tried other cheaper brands like Shiruba and Atman, may not be nearly as quiet or energy efficient, but they work well too.
With your current fish bio-load of 100+ tetra in a 50cm tank, you'll definitely need alot more filtration to handle that kind of high bio-load in the tank, therefore a canister filter would really help alot.
As for the lily pipe sizes, it depends on the hose size that the canister filter uses, usually most medium-sized canister filters use 12/16 hoses so you will pick the 13mm lily pipes (17mm lily pipes are for the larger canister filters which use 16/22 hoses).
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