You should move over all substrate, decor and basically anything that is established in the 75 gallon at the same time you move the fish (filters especially). I would not add any fish for a while despite the tankmates because your 125g is going to have to cycle so adding bioload of more fish will only hurt. Not knowing your budget, I prefer the fluval FX5 canister on a 125g myself but those run about $250 with media. You can safe a ton of $$$ and buy a few inexpensive sponge filters if your on a budget.