MMCGill,
(1) Decide what type of system they want: separate components (easy to upgrade), integrated amplifier and speakers (simpler but more expensive to upgrade), or even all in one speaker systems (Buchardt Audio A10, Dutch and Dutch 8C, KEF LS X, 50 & 60, Sonus Faber Duetoo, etc.) Once I'm done with my main system next year some time, I'll add a second system.....and it'll probably be a Dutch & Dutch 8C or Buchardt A10, as to what is currently available.
(2) Get to know GIK Acoustics. They have excellent room treatments, many of which are not just functional, but also attractive. They also have some software recommendations to help with designing the room treatments, and are all to helpful and freely give advice
(3) Cables matter, but no need to go crazy unless your system is extremely resolving. Morrow Audio, DHLabs, Supra Cables, Lavri Cables, Cullen Cables, Zavfino, AntiCables, all make great cables at affordable prices.....and are great for all but high end systems
(4) Buy ONLY what you can afford, really really good systems can be had without breaking the bank
(5) Audition, audition, audition. Borrow units from dealers and friends, or take advantage of test drives, even "direct only" manufacturers have generous test periods and/or small restocking fees.
(6) Unless you have a stack of LPs or CDs sitting around, just start and stay with streaming. Streaming has come a long way over the last 10 years, and provides highly satisfying SQ. Tips: dedicated streamer and NOT PC based, wire directly to ethernet if possible, quality matters...Wiim Pro, Eversolo DMP-A6, and BlueSound can be excellent though Aurender/Lumin/Innuous do deliver noticeable sound improvements
(7) Have fun, this is a great part of our hobby; it should be a fun time