Have you considered getting a canister that you will use on your 180g and using on your 55g for the time being?
1) You'll have less equipment to buy when you set up your new tank.
2) You'll have a cycled filter to help jump start the new 180g when you set it up. (mini-cycle vs full cycle)
3) Anything you add now will help make sure you have fish still alive when you do set up the larger tank so a nice canister filter cannot hurt.
4) You will not likely be able to find a canister for $40-$50. You can post a WTB add here on MFK and on CL and see if someone bites but you more than likely will have to pony up some cash and get a decent filter. (see point 1 & 2 above as they could help minimize this as an issue for you)
1) You'll have less equipment to buy when you set up your new tank.
2) You'll have a cycled filter to help jump start the new 180g when you set it up. (mini-cycle vs full cycle)
3) Anything you add now will help make sure you have fish still alive when you do set up the larger tank so a nice canister filter cannot hurt.
4) You will not likely be able to find a canister for $40-$50. You can post a WTB add here on MFK and on CL and see if someone bites but you more than likely will have to pony up some cash and get a decent filter. (see point 1 & 2 above as they could help minimize this as an issue for you)
hahahaha 
If your buddy's tank is in storage what the harm in adding them to the tank you have an cycling them/filtering your current tank? As far as the salesman, like others already said, you need to invest in a liquid test kit for yourself and determine what you need. You will need one when you set up your new tank anyway.