Loaded question.. there are many threads on here about it as well. I'll define 'need' as minimum. At a minimum all you need is somewhere for biological, mechanical and possibly chemical filtration to take place. That could be HOB
filter(s), canister filter(s), live rock, etc. Even sand is optional, barebottom tanks are not out of the question. Throw some lights on the top and (after cycling) you have a FO
That's the minimum you need to keep a good selection of saltwater fish. Though you will be limited to the types of fish since some need sand and/or live rock.
If the guy is also giving you his filters more the better. Use them if you want.
You can always buy a HOB
skimmer as well or go full blown and use an overflow, sump and in-sump skimmer.. or you can wait till down the road. It's your $$ and your time.
O yea, and a heater (or preferably 2)