I'm completely new to this so please bear with -Welcome. Espresso is quite a rabbit hole to fall down!
I had a Gaggia Classic as my first machine and it served me very well. As far as getting a better grinder than the machine I would say -
1. The Classic can produce GREAT coffee you can add a PID to improve temp stability and play with different temperatures to extend it's 'life'.
2. If you go for a good grinder now it could easily serve you if you did decide to upgrade to say a nice HX machine in the future and doing it like that would be a lot cheaper in the long run.
The Niche is a great grinder from what I read. Another grinder at a more reasonable price point to consider is the Eureka Mignon Specilita, it is a very popular choice due to its quality and performance. I can do you one for £355 if you are interested.
what does PID mean?
im about to buy my first machine, looking at either sage barista express or the pro. Any advice?
The gagia classic seems to be very popular. I wouldn't be able to afford one of these plus a niche.....& a second hand machine, I'm not really technical so not sure I'd be able to take it apart...I have no idea what shower screens are etc!