Should I buy a a3 all in one printer
I wouldn't personally call the ET-2550 an all-in-one. With no document feeder, your scans and copies must be a pain. The choice is a matter of personal preference, of course, but I tend to recommend getting the single-most versatile printer you can get. In my experience, it's not a matter of if, but when you will want to scan at A3, or print a poster, or print a photograph.
And if you're doing mostly scanning/copying now, then do yourself a favour and get a printer/scanner with a document feeder and that can scan both sides of a page. Look at some of the Brother inkvestment tank printers. MFC-J6945DW, for example.