Atomic M1 is a great reg but insanely expensive and I don't have any love for unsealed pistons
The M1 is a sealed piston. Cost is $830 for a single 1st/2nd stage combo.
A ScubaPro Mk25/G260 is $1000.
A Zeagle F8 is $650.
A Dive Rite XT1/XT4 is $539.
By my personal standards, the Atomic M1 does not qualify as "insanely expensive", but that is just me. Having all of those listed except the Zeagle, the M1 would be (and is, in fact) my choice.
The M1 is not an unsealed piston unless you unseal it yourself or have a reg tech leave it unsealed after servicing. The servicing of an M1 would be expected to cost a bit more than servicing other 1st stages because of the christolube used to seal it. Personally, I'm okay with that fairly small extra expense. Especially considering how infrequently the M1 needs to be serviced. I've had my doubles set of M1s for 3 or 4 years now and they have not yet shown any sign of needing to be serviced. So, call it an extra $50 every 3 years (or more) for service? It's worth it to me for how well they breathe and how high quality they are.
Yes, the M1 is more expensive than some other fine options (and less expensive than other fine options - I do like my ScubaPro Mk25 EVO/G260 regs also). Whether it's worth the extra expense to anyone else is really up to them and their wallet.
As for a BP/W, the back plate part is simple. Back plates made of stainless steel are pretty much the same. There are minor differences in finish - but I couldn't in good conscience suggest anyone pay an extra $100 for one over another because of the finish. There is nothing bad to say about a DGX back plate. Just make sure whatever you buy has 2 slots for cam bands - so you have the choice of using an STA or not using an STA if you don't want to. Almost all BPs do have those these days, but maybe not ALL.
Wings and harnesses are another matter altogether.
I have seen Hollis wings and had students with Hollis wings. I would not buy one or recommend one. They are not terrible. But, there are definitely better options, in my opinion.
My favorite wings, of what is currently in production, are Dive Rite wings. They perform really well, are well made, are reasonably priced, and are supported by excellent customer service. The DGX single tank wing is also good quality and a good performer.
For a harness, I would not buy any harness that anybody makes. I just built a brand new BP/W for a nephew. What I did for him is what I would do for myself and would recommend to anyone. Buy 12' or so of 2" webbing, bent D-rings, tri-slides, a buckle, 2 cam bands, some thin bungee, some sex bolts (book screws), and a crotch strap and put my own together.
The result is not just less expensive than a pre-made harness. It is BETTER - in my opinion. Nothing plastic (that can break). No padding (that does nothing but get in the way when you want to adjust stuff, and make stuff more floaty). Nothing that can't be fixed or replaced very quickly, easily, and cheaply - but will not need to be.