When stepping up to bigger tanks its part of the deal.. The bigger the tank the more $$$ things cost. Especially filtration. Its also the LAST place you ever wanna skimp. Personally I HIGHLY recommend to have a filtration turn over rate of 8 to 10 time the tank size. I gave up on HOB filters years ago. They do not have the volume or capcity of filtration as a Sump/wet dry or as much as a Canister. Flow rates in HOB's I think are highly over rated. Take your time be patient in your choice here. If you dont your going to end up spending way more then you really needed just to upgrade later.
Canisters are superior to HOB's because a quality rated canister for the right turn over will always out flow the HOB. Also a Canister can hold TONS more Biological filtration along with mechanical filtration. Canisters are also nice as far as reliability and use. As well are concealed easily.
For a 125 gallon tank you need roughly 800 to 1000 gallons per hour. The best canister for that job is the Fluval FX5. It comes in at roughly 900 to 1000 GPH. its huge with tons of media capability. Is it $100 or less? No.. Its about $250. BUT worth it.
Now..
Sumps/Wetdry filtration is superior to Canisters for max filtration, use and creativeness to do things with. If you go and pick yourself up a used Wet dry or a Old 29 gallon tank and convert it to a sump filter youd be set.
Based on what im reading from your posts, I think you should budget, Wait and save the money and either do the Sump or a Canister filter like an FX5. There is NO HOB that can compare. I honestly think going cheap is not in your best interest.
