hello and welcome! you are in the right spot!!! .... but maybe not the right spot in the spot ..... I would suggest posting all your questions, including equipment questions, in the "new to saltwater and reef" subforum. I'll make you a link ....
https://www.reef2reef.com/forums/new-to-saltwater-reef-aquariums-post-here.130/
there ... put all your questions there as we'll know to give answers appropriate to someone who is new to saltwater. But we LOVE questions, ask away!!
as far as your question of using the same equipment ..... what do you already have?? give us a list of everything you have already and we'll give you our thoughts. (Be specific about what you have ) But you are doubling your water volume so keep that in mind
Used tank: Craigslist!! (where I got mine) or you can look for an online forum like this one that is specific to your area. Google "reef aquarium forum" and the name of your state. Even here there is a subforum of local groups where you can get a pointer in the right direction. You are probably going to pay a little more on craigslist - let us know here the deal you are looking at and we can give you some idea if it sounds reasonable. But if you find a local forum, you should be able to get excellent price there. Those people will be getting out of the hobby or upgrading and they know what stuff is truly worth. But if you are going for a 180, you are going to be looking for someone getting out of hobby. Where are you? if near Michigan, I know of a few posts that say they are looking to sell a 150 and I think a 180
Welcome again!!!