a little story first. about 20 years ago, my family was camping and ended up near another camper who turned out to be one of the higher-up's within home depot. at the time, my dad had around 15 years of electrical experience, and wasn't very happy with where he was. one thing led to another, and they started seriously talking about fast-tracking him to be the head of the local home depot electrical department.
the talks were going great until they got to the pay. they refused to pay him more than $18/hr, with only the potential for yearly cost-of-living increases, which was a fraction of what he was making as an electrician. he didn't take the job.
home improvement stores don't hire people that know what they're talking about because the people that have the valuable experience cost too much.
you're putting way too much faith into a home improvement store employee that's getting paid a low wage, assuming he's keeping up to date on something that largely doesn't affect him in any way.
100A should be more than adequate. the more important aspect here is to make sure that you're getting a large enough 100A panel. a very rough count on circuits puts you at around 14 circuits based on the information given(oven 220, washer 220, water heater 220, ac[definitely do a 20A circuit for this], fridge, kitchen 1, kitchen 2, bathroom, lighting[didn't specify, but going to need it, could be part of a gen purp circuit], gen purp 1, gen purp 2), so i would go no less than a 20-space panel.
isn't there also a water heater that must be installed? i assume that'll be electric as well--my assumption would be 220/20A.
as far as electrical code, i highly recommend you start with something like this, as it's easier to understand then the national electrical code books. and it's got pretty pictures:
https://smile.amazon.com/gp/product/...?ie=UTF8&psc=1
i have a copy of that, specifically because one inspector i've had to deal with approves/deny's all work based on those pictorals, despite some of the pictorals not being 100% accurate for the situation.
first and foremost, it does not cover everything. it paints with a very wide brush. generally speaking, following what's in there as a guide will be code complaint. but it does not account for any local building code amendments.
the other thing i need to bring up is AFCI protection. 2014 code fully implemented AFCI protection on almost all circuits that are not GFCI protected. this varies widely from place to place. part of illinois where i work is on 2014 code but then i also work in indiana, who is on 2008 code, but struck AFCI's from their approved code due to teething issues with the new technology when they originally tried to add AFCI protection from 2002 code and it turned into a major fiasco.
my illinois/indiana experience is unique, as indiana is a state-ran code, meaning all of indiana is on 2008 nec code. the local inspectors can't ask for anything beyond what is required in 2008 nec code. but illinois, while officially on 2008 code, is a township system, meaning that each township has the power to adopt newer versions of the code, or institute their own specific additions, which is legal as long as their additions go above and beyond the version of code they are on. it means that there's dozens of tiny rules in various towns that don't get applied anywhere else, but still need to be followed.
you can see by this page how versions of code change from state to state, so if you're working in multiple area's the rules can be different, but notice that illinois says it's on 2008, while part of my experience knows that is only partly correct.
https://www.nfpa.org/NEC/NEC-adoptio...-adoption-maps
to your question about sharing a circuit for the kitchen and bath: no. the kitchen must have 2-20A dedicated circuits minimum. it's written in the code that these circuits cannot serve any other outlets. and the same goes for the dedicated 20A bathroom circuit. it must only serve the bathroom outlet, and no where else. code does allow multiple bathroom's to be served by the same 20A circuit, but that is not a situation i see you doing.
as far as "doing everything with 20A" it's a very homeowner thing to do. sorta "if a little gasoline on a fire is good, dump the whole things on there to be better!" thinking.
general purpose outlets are exactly that, general purpose. think on how you use the outlets within your own residence. to charge a phone here, a laptop there, maybe plug in a light or two. the general rule of thumb i go by is roughly 10 openings(lights or outlets) per circuit.
forums can be your best friend
https://www.mikeholt.com/forum/Forum1/HTML/003753.html
12 gauge is more expensive than 14 gauge. and overall, the only real benefit is slightly more ampacity within the circuit(overall openings count doesn't change), and reducing the physical material you would need to get to perform the job.
per code, any residential 15A or 20A branch circuit can have 15A outlets on them, so there's no real reason to pay extra for 20A outlets unless you're really feeling special. the only time a 20A outlet is required is for dedicated appliances-- like a water heater, or an a/c unit.