I would disagree with pretty much everything written here. YTS 62 or TW01 are easily good enough for professional use. They would be vastly more expensive than the options being pursued by the OP. Playing for 18 months does not dictate that you should splash out nearly £3k on a new sax. Second hand buying of saxophones is no problem. Over the last 40 years I've had about 150 saxes, only one bought new. No problems with them, and almost all sax players have played a used instrument. Saxophones do need care and attention but they are not 'complex machinery'.Becasue you’ve been playing for 18 months, buy new the YTS62 or TWO1. Both are decent saxes, although not true professional instruments, they are good enough to get out of the way up to grade 7 or 8.
I would advise against secondhand. Saxophones are complex machinery that need to be handled gently. A well serviced example second hand can quickly lose its edge in a few months and reveal underlying issues.