Don't do a recessed light above the sink, you will get shadows. Don't do ONLY a recessed light above the sink. I also used pretty glass tile in the showers and it looks really nice when illuminated with 2 lights and dimmed just a bit. Ambiance lighting for when guests use the full bath downstairs! That sounds so nice! Two additional thoughts, not related to the above note: As you plan your lighting, consider your color scheme. If you're going with today's trendy lots-of-white, maybe-some-gray color scheme, light's going to bounce off those stark whites and could feel harsh. On the other hand, if you're using a softer, darker color scheme, those darker colors will "absorb" some of the light; so you'd need to ask whether you'd need more light fixtures. I like to read in the tub, so I definitely plan to have a light of some sort "behind me" for that purpose.

A double light over each sink should be sufficient with a window and assuming the shower is directly across from the vanity and that is the extent of the room. Other options are three single sconces mounted on the mirror and an over head light to give more light to the shower. I have a four-light over a 60" vanity in a 5x11 windowless room. The vanity area is well-lit. The shower at the end of the room is a little more shadowed but not enough to make showering and cleaning an issue.

1. Your plan. 2. My two cents, yes. 3. Do you feel you need any bathroom lights on a dimmer switch? I rarely take baths, and when I do I like to read in the tub, so a dimmer switch isn't necessary. But I know some people like to relax entirely, and want it dark with candles. So maybe a dimmer switch would help for that. Also, given the proximity of the bedroom, would a dimmer switch on the one general overhead light help it to serve as a sort of nightlight? Aside from those two scenarios (and I could be missing others!), I can't see why a dimmer switch would be necessary. And if you're reasonably handy, it's something you could easily (and more cheaply) add later, esp if you're not convinced of its utility right now.

If one of the two baths is an en suite devoted to one bedroom, I would say it's worth adding at least a half bath. If you have the space, go for a full bath. An extra toilet is nice and even just a small standing shower can help make your home run more smoothly when all of the bedrooms are full. I don't know the exact trade-off on resale. There are a lot of factors that will determine that, including the finishes you choose in the bathroom, the cost of the work in your area, and the overall impact the bathroom has on how your home flows and fits together. A bad reno can be worse than just leaving the house as is, so the question of "will it help resale?" can be really tough to answer without managing the project every step of the way. Example: Removing the master bedroom's walk in closet to add a bathroom or adding a bathroom that opens directly into your once fabulous kitchen.... Probably won't have the desired trade-off. Those are some decently obvious pitfalls, but there are many, many other less obvious ones that would only come up on a case-by-case basis. That said, I've been doing a lot of research on bedroom:bathroom ratio. It isn't an exact science, and I didn't crunch hard data, but anecdotally, this is the estimate I come up with. If n is the number of bedrooms.... The number of toilets should be at least n-2. ... 4-2 = a 4 bedroom home should have at least 2 toilets. The number of toilets in the home gains little above n+1. ... 4+1 = a 4 bedroom home gains little value from having more than 5 toilets. So, having between 2 and 5 toilets is good for a 4 bedroom home. The home loses sell-ability with only 1 bathroom. The resale increase diminishes for more than 5. The number of showers in the home should be at least n-2. ... 4-2 = a 4 bedroom home should have at least 2 showers. The number of showers in the home gains little above n. ... 4 = a 4 bedroom home gains little value from having more than 4 showers. So, having between 2 and 4 showers is good for a 4 bedroom home. All homes should have at least one bathtub. If there is only one bathtub in a home that has at least 3 bedrooms, that bathtub should be kid friendly (most people buying 3+ bedroom homes are parents or people planning to be parents). I haven't figured out a rule of thumb for when it becomes necessary for there to be a second bathtub, but at some point, more bathtubs become essential to provide the luxury that fits the home. Hope this helps.