The 100a on the BMS is the rating of the max continuous draw, so a 100a BMS on a 12v battery could feed about a 1Kw inverter before you start stressing the BMS. The 100a on the charge controller is the maximum amperage it could send to your battery bank. There's a difference, and sometimes the BMS will charge at a lower rate than it can discharge, but that's becoming more and more rare. For reference I used a 120a BMS for both of my battery builds just to get that little extra out of them since the BMS's were pretty much the same price between 100a and 120a.
As to the panels, the rule of thumb is Panel Size / Nominal Voltage = Amps of SCC, so 400w of panel / 12v = 33a, so a 40a is plenty. If you wanted more charging capacity you'd need a larger SCC. If you did a 60a SCC for example you could get 720w of panel, or 800w of paneling with inefficiency and imperfect weather would be safe.
With the generic figure of 4 hours of USABLE sun on a 400w array is 1600 watt hours potential generation. 1600 W/h on a 12v system is 133Ah you could theoretically generate. With a 320Ah battery I would recommend at LEAST 60a of charging with 720w of panels (800 for round numbers) which would give you 4hr * 60a = 2400Wh / 12v = 200Ah potential power generation in a day, give or take.
If you really wanted to be able to completely drain and recharge in 1 day the math gets bigger:
320Ah * 12v = 3840Wh. 3840Wh in 4 hours = 960w of panel (call it 1000 for round numbers, weather, inefficiencies). 1000w of panel / 12v system = 83A SCC to feed that. Now, in the Real World you'd be lucky to get 80A to the batteries so a pair of 40a SCC's in parallel to the battery each with its own 500w worth of panels would pretty much max out that system.
Now, if you can live on less than 130Ah or 1560 watt/hours a day then you wouldn't drain your battery any faster than a 400w array could refill it, but if you got multiple days of krappy weather you'd have to spend a lot of time playing catch-up.
Remember, these guides were done when the most common size cell were 100Ah cells so a 400w system could refill a 100Ah cell in an average day. Your battery is 320% larger so the size of the charge controller and panels will need to scale up accordingly.
As for parts for this whole system, the 2 most common ways are:
1: The "Easy" way - An
MPP 1012LVM All-In-One inverter, a Class-T 125a fuse, a
120a DC breaker (for disconnecting), inline fuses for panels OR a
combiner box with fuses.
Pro's: Cheap, easy to assemble, has all the working parts in a single package.
Con's: About 10% of your battery capacity will go to the AIO unit to exist, only provides 1000w of 120v AC output, only a 40a SCC built in so you'll need another SCC and array in parallel to fully charge the batteries in a day, one part goes out replace the whole unit.
2: The "Custom Way" - A decent
quality 1000-1200w Pure Sine Inverter, dual
40a MPPT SCC's, a Class-T 125a fuse, a
125a DC breaker (for disconnecting), inline fuses for panels OR a
combiner box with fuses.
Pro's: Slightly less standby power, maxes out the realistic charging capacity of your system, individual pieces can be replaced, more clout on your favorite solar forum.
Con's: Much more expensive than going with the AIO route, takes up more physical space, you'll get good at crimping cables.
If you're running anything off a DC bus like lights or diesel heaters or the like, you'll need to get a
DC fuse block involved, and that's more based on how many slots you'll need than anything else.
If you're working with a tighter budget there are places you can and can't skimp out on to work with that. The SCC doesn't have to be Victron levels of $pendy, a Rich Solar, HQST, or even a PowMr will turn solar DC into battery DC for much less money. The MPP 1012 unit is very cost effective when you look at all the things it includes like the 40a SCC, the inverter, the generator input, the automatic transfer switch, and the convenience factor. The place you DON'T want to skimp are your safety devices like the fuses and breakers. Remember, fuses are (often) cheaper than fires!
Having said that, for some reason 120a Class-T fuses are harder to find than Unobtanium or an honest politician so that's going to be a challenge. I personally gave up after months of hunting and just used a good quality DC breaker and took the risk, but YMMV.
Used panels are a great deal if A: you can find them locally and B: you've got lots of space to put them. Shipping costs on panels means that unless you're buying 2 dozen of them, it's getting pretty even on the $/watt scale to new panels from Amazon where you're looking in the neighborhood of $1/w shipped. Lots of people will tell you "I can get used panels for $0.10/watt all day long" but when you start at $400 shipping PLUS panels, that figure falls apart pretty quickly if you're only getting 4 panels or so. Craigslist and FB Market are your friends, but sometimes Amazon is the winner.
Also, know the limitations of your system. A 12v battery with a 100a BMS can't feed more than about 1200w so don't be tempted to buy that Chinesium 5000w 12v inverter, it's junk. Also, spending the money on a Pure Sine rather than a cheaper Modified Sine will pay off in the long run. Sure, it's cheaper now and you can take all that money you saved and put it towards new appliances or electronics that the MSW inverter fried. What a value!
Have you done a power audit yet to guesstimate what your cabin is going to use yet? It'll help give you an idea of what to expect your system to accomplish. Like many of us on here before you, rather than making a system to do what you need, you're starting with a system and seeing what it can do. Not horrible, but there is a "Tuition" fee to buying parts first and hoping you can use them all later.
Don't worry, you're not the first and you won't be the last. ?