Sounds like a nice update! Since you have two guest/kid bathrooms, I think the one big rectangular sink is perfect. The storage space provided by the drawers is invaluable IMO.
Moderate renovations have better return on investment than over the top ones, so keeping the fixtures in the same place and doing the updates you mentioned is the way to go. Best of luck!

I would do 2 sinks in that room. You have to remember who might want to live in your house in the future, even though your kids were fine sharing, a small thing like that (since you’re updating the whole vanity anyway) could help sell it. You have a large enough vanity to do it and it would be more visually appealing.

I wanted to add, that since your current vanity is pretty simple and white, if it's in good shape you could keep it. Just changing the countertop, sink, mirror and light (can't see them!) would transform the current vanity and probably cost less.

A realtor told me that a light bathroom update will give you a better return than a gut job. Buyers will see a fresh, clean and bright bathroom without thinking “we’ve got to update this.” We kept our vanities in all 3 bathroom projects because the boxes were strong and there was no reason to demolish them. But the doors were in bad shape. So we had a local cabinet builder make us new doors and drawer fronts. Each bathroom got new doors and drawer fronts, sink, faucet, countertop with simple back and side splashes of counter material, lights, tp holder and towel ring, towel hooks or bar, shower curtain, some cute decor, small storage cabinet over toilet for TP, and re-painted. One bathroom got a new mirror. At that time we left the floors, tubs and showers alone.

Good luck!

Forgot to say:

We also got new cabinet hardware.

Everything came from HD, or Lowe’s. A lot of times, the exact same products are at HD and Lowe’s, but one store will have the item at a discount and not the other. For example I wanted a particular flush ceiling light and over-vanity light for one bathroom. Lowe’s had the ceiling light for $39, and the vanity light for $150. HD had the ceiling light for $79, and the vanity light for $59. So I bought the ceiling light at Lowe’s, then drove to HD for the vanity light. That same scenario occurred for a lot of the products, enabling us to come in under budget.

I would ask your realtor. I had a bathroom with one sink between two bedrooms and my realtor suggested I change the vanity to a two sink vanity. The other two bedrooms upstairs each had their own bathrooms already. But it really depends on the real estate market in your area.